add another error message for the new error.

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7383#note_18518792

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7383#note_18518792

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7383#note_18517263

* upstream/master: (236 commits)
  Mention Git strategy none
  Remove ToC since it's now supported in the docs portal itself
  Add 8.14 to versions with further additions to review apps
  Add Limitations sections to environments and review apps docs
  Add link to environments docs
  Fix URL to review apps docs
  Add a prerequisites section, add some links
  Link to NGINX example project for the time being
  Get rid most of the irrelevant sections
  Add note about current limitation in $CI_BUILD_REF_NAME
  Add an intro and an Overview section for Review Apps
  WIP review apps
  Add Review apps link to CI README
  Add stop environment permissions and remove delete
  Add note about auto-stopping of environments
  Update CHANGELOG.md for 8.13.6
  Finish "Stopping envs" and "Grouping similar envs" sections
  Highlight first user autocomplete option
  Remove ToC and fix headings in Markdown docs
  Revert "Merge branch '22680-unlabel-limit-autocomplete-to-selected-items' into 'master'"
  ...

Mention Git strategy none

See merge request !7530

[ci skip]

Highlight first user autocomplete option

See merge request !7520

Add documentation for Review Apps

## What does this MR do?

Add docs for review apps.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !6982

Allows configuration of idle_timeout for incoming email.

https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1087

See merge request !7423

Improvements to Redis HA docs

Following https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6471

See merge request !7502

[ci skip]

[ci skip]

Remove empty db fixtures file

## What does this MR do?
Removes empty fixtures file

See merge request !7519

[ci skip]

[ci skip]

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Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

[ci skip]

Refactor environments documentation

## What does this MR do?

Refactor environments docs.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !7107

[ci skip]

[ci skip]

[ci skip]

Stop environment when branch is deleted

## What does this MR do?

This MR adds a environment teardown service, that is called when user deletes a branch. This most often happens when merge requests is merged.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

## What are the relevant issue numbers?

Closes #23218

See merge request !7355

Update CONTRIBUTING.md - Adding info about issues within a milestone

## What does this MR do?
Adds copy regarding issues within the current milestone to the contribution guide.

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @dzaporozhets @awhildy

See merge request !7516

Revert "Merge branch '22680-unlabel-limit-autocomplete-to-selected-items' into 'master'"

There were some issues with the initial implementation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796#note_18557076

This reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796

See merge request !7521

Remove ToC and fix headings in Markdown docs

See merge request !7523

Adds hash anchors to diff_files to make possible linking exact file in commit

Closes #24010

See merge request !7298

[ci skip]