Refactor the upgrading from source docs

Closes gitlab-org/release/framework#190

See merge request gitlab-org/gitlab-ce!25003

Update on using self-signed certs with registry

See merge request gitlab-org/gitlab-ce!25086

Fix branch API table format

Closes #57480

See merge request gitlab-org/gitlab-ce!25091

This changes the guides for upgrading from CE source to EE source so we
no longer need separate documents for every version. This removes the
need for release managers to create and update these documents, and
allows developers to add these instructions as part of their code
changes.

Remove HipChat integration from GitLab

Closes #52424

See merge request gitlab-org/gitlab-ce!22223

Instead of requiring one separate document for every version (which is
created by release managers), we now use a single document for both
Community Edition and Enterprise Edition. This allows developers to add
guidelines right away, instead of release managers having to determine
what to add.

Version specific upgrading instructions are placed in a separate
section. This prevents instructions from lingering around for many
versions should a developer forget to remove them. This also ensures
that all instructions are kept in a single place, instead of being
spread across different documents.

A minor downside is that CE and EE guidelines now live in the same
document, which could cause merge conflicts. Since we are working
towards a single codebase, this should only be an issue until we merge
the codebases together; something we expect to do in the coming months.

This commit also removes all old upgrading instructions, but these can
still be accessed using Git branch specific URLs (included in the
updating README).

Add missing argument to DeploymentService#predefined_variables

See merge request gitlab-org/gitlab-ce!24989

Directly inheriting from ActiveRecord::Migration is deprecated

See merge request gitlab-org/gitlab-ce!25066

use timeout as it appears in logs

See merge request gitlab-org/gitlab-ce!21264

Validate k8s CA certificate at cluster creation

See merge request gitlab-org/gitlab-ce!24990

Use correct integrity hash for sprockets caching

Closes #57431

See merge request gitlab-org/gitlab-ce!25033

[CE] Improve `wait_for_push`

Closes #53308

See merge request gitlab-org/gitlab-ce!24085

Provide a performance bar link to the Jaeger UI

See merge request gitlab-org/gitlab-ce!24902

Externalize date picker string

See merge request gitlab-org/gitlab-ce!25068

Docs: correct word

See merge request gitlab-org/gitlab-ce!25065

Fix create_project_spec

Closes gitlab-org/quality/staging#32

See merge request gitlab-org/gitlab-ce!25029

Removes milestone from cluster base domain documentation

See merge request gitlab-org/gitlab-ce!25045

Fix error when reloading code in Sidekiq

Closes #54320

See merge request gitlab-org/gitlab-ce!25069

This commit fixes the following error in development:

    ArgumentError: A copy of Gitlab::Metrics::Transaction has been
    removed from the module tree but is still active!

Resolve "ActionView::Template::Error: undefined method `>' for nil:NilClass"

Closes #57428

See merge request gitlab-org/gitlab-ce!25028

Adds a feature flag to the diff fuzzy file finder

Closes #57403

See merge request gitlab-org/gitlab-ce!25006

12.0 milestone was removed from cluster base domain on AutoDevOps
documentation to avoid making fake promises.

Update `chart.js` to 2.7.2

See merge request gitlab-org/gitlab-ce!24771

Fix push_http_private_token_spec by waiting for viewers to load

Closes gitlab-org/quality/staging#33

See merge request gitlab-org/gitlab-ce!25030

Mention when the Registry API was introduced

See merge request gitlab-org/gitlab-ce!24596

Fixes diff web worker not loading in staging

See merge request gitlab-org/gitlab-ce!25039

Clarify search parameter in the branches API docs

See merge request gitlab-org/gitlab-ce!24703