For 4 years we've had logic in our database migrations that required you
to set a constant `DOWNTIME = true` if you required downtime and seek
approval from the VP of Engineering. We have never once used this
process as we've always found a way around the problem using a different
approach.

As such we decided in
https://gitlab.com/gitlab-org/gitlab/-/issues/326495 that we should just
remove this `DOWNTIME` constant and the extra checks here to reduce
noise in our database code review and give less processes for people to
learn.

This MR removes a lot of things and here is a high level summary:

1. Remove DowntimeCheck class and the rake task that invoked and the CI
job that invoked that rake task and any related tests, helper classes
2. Update documentation to make it clearer that downtime is not allowed
and therefore remove the approval process
3. Update a page called `what_requires_downtime` to be called
`avoiding_downtime_in_migrations` since it was already a set of patterns
to avoid downtime and now it's worded more strongly to imply that we can
always get away without downtime
4. Various other docs pages that had examples of migrations that used
the `DOWNTIME` constant
5. Various rubocop specs that had migrations in them which used the
`DOWNTIME` constant

The one thing I did not do is go back and remove `DOWNTIME = false` from
all the existing migrations. In general we should not be updating
migrations once they've run and this would have made this MR change many
thousands of files so it's not worth it.

Suggestions on a Merge Request | E2E & Cleanup

See merge request gitlab-org/gitlab!57810

Provide option to disable project access token creation [RUN ALL RSPEC] [RUN AS-IF-FOSS]

See merge request gitlab-org/gitlab!57756

Fix Rails/SaveBang Rubocop offenses for issue models

See merge request gitlab-org/gitlab!58052

Remove feature flag optimize_issue_filter_assigned_to_self [RUN ALL RSPEC] [RUN AS-IF-FOSS]

See merge request gitlab-org/gitlab!57775

Standardize project-level header for MR approvals

See merge request gitlab-org/gitlab!58116

Include links to otherwise unlinked Manage group docs

See merge request gitlab-org/gitlab!58461

Suggest debug logging when troubleshooting SAST

See merge request gitlab-org/gitlab!58538

Move to confirm variant for buttons in vulnerabilities page

See merge request gitlab-org/gitlab!57961

Reduce use of master in Secure docs

See merge request gitlab-org/gitlab!58602

Updated renew steps

See merge request gitlab-org/gitlab!57894

Provide `can_admin_list` to the boards app  [RUN AS-IF-FOSS]

See merge request gitlab-org/gitlab!57795

- Include can_admin_list in the data returned by
boards_helper

- Add missing spec coverage for remove list button
(board_settings_sidebar_spec.js)

Fix trailing whitespace

Provide can_admin_list for epic boards

Add documentation for Gitaly pack-objects cache

See merge request gitlab-org/gitlab!57371

Fix EmptyLineAfterFinalLetItBe Rubocop offenses for groups controller

See merge request gitlab-org/gitlab!58174

Fixed awards smiley alignment

See merge request gitlab-org/gitlab!58186

Externalize stings in _create_chat_team.html.haml

See merge request gitlab-org/gitlab!58328

Externalize strings in _group_admin_settings.html.haml

See merge request gitlab-org/gitlab!58331

Subscription Details: fix table styles and skeleton

See merge request gitlab-org/gitlab!58375

Step 1 - Create Feature Flag for reject_unsigned_commits_by_gitlab [RUN ALL RSPEC] [RUN AS-IF-FOSS]

See merge request gitlab-org/gitlab!58453

Fix more mis-capitalizations of Merge Request

See merge request gitlab-org/gitlab!58445

Fix RSpec/EmptyLineAfterFinalLetItBe for ee/spec/services/groups

See merge request gitlab-org/gitlab!58343

Point users to Vulnerabilities GraphQL API

See merge request gitlab-org/gitlab!58080

Replace deprecated button on new epic creation form

See merge request gitlab-org/gitlab!57874

Add the user update API endpoint to the Diffs app

See merge request gitlab-org/gitlab!55930

Fix RSpec/EmptyLineAfterFinalLetItBe for ee/spec/services/geo

See merge request gitlab-org/gitlab!58348