Move the Omnibus update doc to the GitLab repo

This is mostly moving the content of
https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/f87042ca218a1f834e17dc769da5eb5f7356aac2/doc/update/index.md
in the GitLab repository, with some minor changes to headings.

doc/update/package/convert_to_ee.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Convert Community Edition to Enterprise Edition **(FREE SELF)**
+
+To convert an existing GitLab Community Edition (CE) server installed using the Omnibus GitLab
+packages to GitLab [Enterprise Edition](https://about.gitlab.com/pricing/) (EE), you install the EE
+package on top of CE.
+
+Converting from the same version of CE to EE is not explicitly necessary, and any standard upgrade
+(for example, CE 12.0 to EE 12.1) should work. However, in the following steps we assume that
+you are upgrading the same version (for example, CE 12.1 to EE 12.1), which is **recommended**.
+
+WARNING:
+When updating to EE from CE, avoid reverting back to CE if you plan on going to EE again in the
+future. Reverting back to CE can cause
+[database issues](index.md#500-error-when-accessing-project--settings--repository)
+that may require Support intervention.
+
+The steps can be summed up to:
+
+1. Find the currently installed GitLab version:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   sudo apt-cache policy gitlab-ce | grep Installed
+   ```
+
+   The output should be similar to: `Installed: 13.0.4-ce.0`. In that case,
+   the equivalent Enterprise Edition version will be: `13.0.4-ee.0`. Write this
+   value down.
+
+   **For CentOS/RHEL**
+
+   ```shell
+   sudo rpm -q gitlab-ce
+   ```
+
+   The output should be similar to: `gitlab-ce-13.0.4-ce.0.el8.x86_64`. In that
+   case, the equivalent Enterprise Edition version will be:
+   `gitlab-ee-13.0.4-ee.0.el8.x86_64`. Write this value down.
+
+1. Add the `gitlab-ee` [Apt or Yum repository](https://packages.gitlab.com/gitlab/gitlab-ee/install):
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   curl --silent "https://packages.gitlab.com/install/repositories/gitlab/gitlab-ee/script.deb.sh" | sudo bash
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   curl --silent "https://packages.gitlab.com/install/repositories/gitlab/gitlab-ee/script.rpm.sh" | sudo bash
+   ```
+
+   The above command will find your OS version and automatically set up the
+   repository. If you are not comfortable installing the repository through a
+   piped script, you can first
+   [check its contents](https://packages.gitlab.com/gitlab/gitlab-ee/install).
+
+1. Next, install the `gitlab-ee` package. Note that this will automatically
+   uninstall the `gitlab-ce` package on your GitLab server. `reconfigure`
+   Omnibus right after the `gitlab-ee` package is installed. **Make sure that you
+   install the exact same GitLab version**:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   ## Make sure the repositories are up-to-date
+   sudo apt-get update
+
+   ## Install the package using the version you wrote down from step 1
+   sudo apt-get install gitlab-ee=13.0.4-ee.0
+
+   ## Reconfigure GitLab
+   sudo gitlab-ctl reconfigure
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   ## Install the package using the version you wrote down from step 1
+   sudo yum install gitlab-ee-13.0.4-ee.0.el8.x86_64
+
+   ## Reconfigure GitLab
+   sudo gitlab-ctl reconfigure
+   ```
+
+1. Now go to the GitLab admin panel of your server (`/admin/license/new`) and
+   upload your license file.
+
+1. After you confirm that GitLab is working as expected, you may remove the old
+   Community Edition repository:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   sudo rm /etc/apt/sources.list.d/gitlab_gitlab-ce.list
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   sudo rm /etc/yum.repos.d/gitlab_gitlab-ce.repo
+   ```
+
+That's it! You can now use GitLab Enterprise Edition! To update to a newer
+version, follow [Update using the official repositories](index.md#upgrade-using-the-official-repositories).
+
+NOTE:
+If you want to use `dpkg`/`rpm` instead of `apt-get`/`yum`, go through the first
+step to find the current GitLab version and then follow
+[Update using a manually-downloaded package](index.md#upgrade-using-a-manually-downloaded-package).

doc/update/package/downgrade.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Downgrade **(FREE SELF)**
+
+This section contains general information on how to revert to an earlier version
+of a package.
+
+WARNING:
+You must at least have a database backup created under the version you are
+downgrading to. Ideally, you should have a
+[full backup archive](../../raketasks/backup_restore.md#back-up-gitlab)
+on hand.
+
+The example below demonstrates the downgrade procedure when downgrading between minor
+and patch versions (for example, from 13.0.6 to 13.0.5).
+
+When downgrading between major versions, take into account the
+[specific version changes](index.md#version-specific-changes) that occurred when you upgraded
+to the major version you are downgrading from.
+
+These steps consist of:
+
+- Stopping GitLab
+- Removing the current package
+- Installing the old package
+- Reconfiguring GitLab
+- Restoring the backup
+- Starting GitLab
+
+Steps:
+
+1. Stop GitLab and remove the current package:
+
+   ```shell
+   # If running Puma
+   sudo gitlab-ctl stop puma
+
+   # Stop sidekiq
+   sudo gitlab-ctl stop sidekiq
+
+   # If on Ubuntu: remove the current package
+   sudo dpkg -r gitlab-ee
+
+   # If on Centos: remove the current package
+   sudo yum remove gitlab-ee
+   ```
+
+1. Identify the GitLab version you want to downgrade to:
+
+   ```shell
+   # (Replace with gitlab-ce if you have GitLab FOSS installed)
+
+   # Ubuntu
+   sudo apt-cache madison gitlab-ee
+
+   # CentOS:
+   sudo yum --showduplicates list gitlab-ee
+   ```
+
+1. Downgrade GitLab to the desired version (for example, to GitLab 13.0.5):
+
+   ```shell
+   # (Replace with gitlab-ce if you have GitLab FOSS installed)
+
+   # Ubuntu
+   sudo apt install gitlab-ee=13.0.5-ee.0
+
+   # CentOS:
+   sudo yum install gitlab-ee-13.0.5-ee.0.el8
+   ```
+
+1. Reconfigure GitLab:
+
+   ```shell
+   sudo gitlab-ctl reconfigure
+   ```
+
+1. [Restore GitLab](../../raketasks/backup_restore.md#restore-for-omnibus-gitlab-installations)
+   to complete the downgrade.

doc/update/package/index.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Upgrade GitLab using the GitLab Package **(FREE SELF)**
+
+This section describes how to upgrade GitLab to a new version using the
+GitLab package.
+
+We recommend performing upgrades between major and minor releases no more than once per
+week, to allow time for background migrations to finish. Decrease the time required to
+complete these migrations by increasing the number of
+[Sidekiq workers](../../administration/operations/extra_sidekiq_processes.md)
+that can process jobs in the `background_migration` queue.
+
+If you don't follow the steps in [zero downtime upgrades](../zero_downtime.md),
+your GitLab application will not be available to users while an upgrade is in progress.
+They either see a "Deploy in progress" message or a "502" error in their web browser.
+
+Prerequisites:
+
+- [Supported upgrade paths](../index.md#upgrade-paths)
+  has suggestions on when to upgrade. Upgrade paths are enforced for version upgrades by
+  default. This restricts performing direct upgrades that skip major versions (for
+  example 10.3 to 12.7 in one jump) that **can break GitLab
+  installations** due to multiple reasons like deprecated or removed configuration
+  settings, upgrade of internal tools and libraries, and so on.
+- If you are upgrading from a non-Package installation to a GitLab Package installation, see
+  [Upgrading from a non-Package installation to a GitLab Package installation](https://docs.gitlab.com/omnibus/convert_to_omnibus.html).
+- It's important to ensure that any
+  [background migrations](../index.md#checking-for-background-migrations-before-upgrading)
+  have been fully completed before upgrading to a new major version. Upgrading
+  before background migrations have finished may lead to data corruption.
+- Gitaly servers must be upgraded to the newer version prior to upgrading the application server.
+  This prevents the gRPC client on the application server from sending RPCs that the old Gitaly version
+  does not support.
+
+You can upgrade the GitLab Package using one of the following methods:
+
+- [Using the official repositories](#upgrade-using-the-official-repositories).
+- [Using a manually-downloaded package](#upgrade-using-a-manually-downloaded-package).
+
+Both automatically back up the GitLab database before installing a newer
+GitLab version. You may skip this automatic database backup by creating an empty file
+at `/etc/gitlab/skip-auto-backup`:
+
+```shell
+sudo touch /etc/gitlab/skip-auto-backup
+```
+
+For safety reasons, you should maintain an up-to-date backup on your own if you plan to use this flag.
+
+## Version-specific changes
+
+Updating to major versions might need some manual intervention. For more information,
+check the version your are upgrading to:
+
+- [GitLab 14](https://docs.gitlab.com/omnibus/gitlab_14_changes.html)
+- [GitLab 13](https://docs.gitlab.com/omnibus/gitlab_13_changes.html)
+- [GitLab 12](https://docs.gitlab.com/omnibus/gitlab_12_changes.html)
+- [GitLab 11](https://docs.gitlab.com/omnibus/gitlab_11_changes.html)
+
+## Upgrade using the official repositories
+
+All GitLab packages are posted to the GitLab [package server](https://packages.gitlab.com/gitlab/).
+Five repositories are maintained:
+
+- [GitLab EE](https://packages.gitlab.com/gitlab/gitlab-ee): for official
+  [Enterprise Edition](https://about.gitlab.com/pricing/) releases.
+- [GitLab CE](https://packages.gitlab.com/gitlab/gitlab-ce): for official Community Edition releases.
+- [Unstable](https://packages.gitlab.com/gitlab/unstable): for release candidates and other unstable versions.
+- [Nighty Builds](https://packages.gitlab.com/gitlab/nightly-builds): for nightly builds.
+- [Raspberry Pi](https://packages.gitlab.com/gitlab/raspberry-pi2): for official Community Edition releases built for [Raspberry Pi](https://www.raspberrypi.org) packages.
+
+If you have installed Omnibus GitLab [Community Edition](https://about.gitlab.com/install/?version=ce)
+or [Enterprise Edition](https://about.gitlab.com/install/), then the
+official GitLab repository should have already been set up for you.
+
+To upgrade to the newest GitLab version, run:
+
+- For GitLab [Enterprise Edition](https://about.gitlab.com/pricing/):
+
+  ```shell
+  # Debian/Ubuntu
+  sudo apt-get update
+  sudo apt-get install gitlab-ee
+
+  # Centos/RHEL
+  sudo yum install gitlab-ee
+  ```
+
+- For GitLab Community Edition:
+
+  ```shell
+  # Debian/Ubuntu
+  sudo apt-get update
+  sudo apt-get install gitlab-ce
+
+  # Centos/RHEL
+  sudo yum install gitlab-ce
+  ```
+
+### Upgrade to a specific version using the official repositories
+
+Linux package managers default to installing the latest available version of a
+package for installation and upgrades. Upgrading directly to the latest major
+version can be problematic for older GitLab versions that require a multi-stage
+[upgrade path](../index.md#upgrade-paths). An upgrade path can span multiple
+versions, so you must specify the specific GitLab package with each upgrade.
+
+To specify the intended GitLab version number in your package manager's install
+or upgrade command:
+
+1. First, identify the GitLab version number in your package manager:
+
+   ```shell
+   # Ubuntu/Debian
+   sudo apt-cache madison gitlab-ee
+   # RHEL/CentOS 6 and 7
+   yum --showduplicates list gitlab-ee
+   # RHEL/CentOS 8
+   dnf search gitlab-ee*
+   ```
+
+1. Then install the specific GitLab package:
+
+   ```shell
+   # Ubuntu/Debian
+   sudo apt install gitlab-ee=12.0.12-ee.0
+   # RHEL/CentOS 6 and 7
+   yum install gitlab-ee-12.0.12-ee.0.el7
+   # RHEL/CentOS 8
+   dnf install gitlab-ee-12.0.12-ee.0.el8
+   # SUSE
+   zypper install gitlab-ee=12.0.12-ee.0
+   ```
+
+## Upgrade using a manually-downloaded package
+
+NOTE:
+The [package repository](#upgrade-using-the-official-repositories) is recommended over
+a manual installation.
+
+If for some reason you don't use the official repositories, you can
+download the package and install it manually. This method can be used to either
+install GitLab for the first time or update it.
+
+To download and install GitLab:
+
+1. Visit the [official repository](#upgrade-using-the-official-repositories) of your package.
+1. Browse to the repository for the type of package you would like to see the
+   list of packages that are available. Multiple packages exist for a
+   single version, one for each supported distribution type. Next to the filename
+   is a label indicating the distribution, as the file names may be the same.
+1. Find the package version you wish to install and click on it.
+1. Click the **Download** button in the upper right corner to download the package.
+1. After the GitLab package is downloaded, install it using the following commands:
+
+   - For GitLab [Enterprise Edition](https://about.gitlab.com/pricing/):
+
+     ```shell
+     # Debian/Ubuntu
+     dpkg -i gitlab-ee-<version>.deb
+
+     # CentOS/RHEL
+     rpm -Uvh gitlab-ee-<version>.rpm
+     ```
+
+   - For GitLab Community Edition:
+
+     ```shell
+     # GitLab Community Edition
+     # Debian/Ubuntu
+     dpkg -i gitlab-ce-<version>.deb
+
+     # CentOS/RHEL
+     rpm -Uvh gitlab-ce-<version>.rpm
+     ```
+
+## Troubleshooting
+
+### GitLab 13.7 and later unavailable on Amazon Linux 2
+
+Amazon Linux 2 is not an [officially supported operating system](https://docs.gitlab.com/omnibus/package-information/deprecated_os.html#supported-operating-systems).
+However, in past the [official package installation script](https://packages.gitlab.com/gitlab/gitlab-ee/install)
+installed the `el/6` package repository if run on Amazon Linux. From GitLab 13.7, we no longer
+provide `el/6` packages so administrators must run the [installation script](https://packages.gitlab.com/gitlab/gitlab-ee/install)
+again to update the repository to `el/7`:
+
+```shell
+curl --silent "https://packages.gitlab.com/install/repositories/gitlab/gitlab-ee/script.rpm.sh" | sudo bash
+```
+
+See the [epic on support for GitLab on Amazon Linux 2](https://gitlab.com/groups/gitlab-org/-/epics/2195) for the latest details on official Amazon Linux 2 support.
+
+### Get the status of a GitLab installation
+
+```shell
+sudo gitlab-ctl status
+sudo gitlab-rake gitlab:check SANITIZE=true
+```
+
+- Information on using `gitlab-ctl` to perform [maintenance tasks](https://docs.gitlab.com/omnibus/maintenance/index.html).
+- Information on using `gitlab-rake` to [check the configuration](../../administration/raketasks/maintenance.md#check-gitlab-configuration).
+
+### RPM 'package is already installed' error
+
+If you are using RPM and you are upgrading from GitLab Community Edition to GitLab Enterprise Edition you may get an error like this:
+
+```shell
+package gitlab-7.5.2_omnibus.5.2.1.ci-1.el7.x86_64 (which is newer than gitlab-7.5.2_ee.omnibus.5.2.1.ci-1.el7.x86_64) is already installed
+```
+
+You can override this version check with the `--oldpackage` option:
+
+```shell
+sudo rpm -Uvh --oldpackage gitlab-7.5.2_ee.omnibus.5.2.1.ci-1.el7.x86_64.rpm
+```
+
+### Package obsoleted by installed package
+
+CE and EE packages are marked as obsoleting and replacing each other so that both aren't installed and running at the same time.
+
+If you are using local RPM files to switch from CE to EE or vice versa, use `rpm` for installing the package rather than `yum`. If you try to use yum, then you may get an error like this:
+
+```plaintext
+Cannot install package gitlab-ee-11.8.3-ee.0.el6.x86_64. It is obsoleted by installed package gitlab-ce-11.8.3-ce.0.el6.x86_64
+```
+
+To avoid this issue, either:
+
+- Use the same instructions provided in the
+  [Upgrade using a manually-downloaded package](#upgrade-using-a-manually-downloaded-package) section.
+- Temporarily disable this checking in yum by adding `--setopt=obsoletes=0` to the options given to the command.
+
+### 500 error when accessing Project > Settings > Repository
+
+When GitLab is migrated from CE > EE > CE, and then back to EE, you
+might get the following error when viewing a project's repository settings:
+
+```shell
+Processing by Projects::Settings::RepositoryController#show as HTML
+  Parameters: {"namespace_id"=>"<namespace_id>", "project_id"=>"<project_id>"}
+Completed 500 Internal Server Error in 62ms (ActiveRecord: 4.7ms | Elasticsearch: 0.0ms | Allocations: 14583)
+
+NoMethodError (undefined method `commit_message_negative_regex' for #<PushRule:0x00007fbddf4229b8>
+Did you mean?  commit_message_regex_change):
+```
+
+This error is caused by an EE feature being added to a CE instance on the initial move to EE.
+After the instance is moved back to CE and then is upgraded to EE again, the
+`push_rules` table already exists in the database. Therefore, a migration is
+unable to add the `commit_message_regex_change` column.
+
+This results in the [backport migration of EE tables](https://gitlab.com/gitlab-org/gitlab/-/blob/cf00e431024018ddd82158f8a9210f113d0f4dbc/db/migrate/20190402150158_backport_enterprise_schema.rb#L1619) not working correctly.
+The backport migration assumes that certain tables in the database do not exist when running CE.
+
+To fix this issue, manually add the missing `commit_message_negative_regex` column and restart GitLab:
+
+```shell
+# Access psql
+sudo gitlab-rails dbconsole
+
+# Add the missing column
+ALTER TABLE push_rules ADD COLUMN commit_message_negative_regex VARCHAR;
+
+# Exit psql
+\q
+
+# Restart GitLab
+sudo gitlab-ctl restart
+```
+
+### Error `Failed to connect to the internal GitLab API` on a separate GitLab Pages server
+
+Please see [GitLab Pages troubleshooting](../../administration/pages/index.md#failed-to-connect-to-the-internal-gitlab-api).