Port of 27501-api-use-visibility-everywhere to EE

See merge request !1267

Override the `ApplicationSetting` default visibility_level setters so
they accept strings & integers for the levels.

Add `visibility` & `visibility=` methods to the
`Gitlab::VisibilityLevel` module so the `visibility_level` can be
get/set with a string value.

CE upstream: Thursday

See merge request !1339

And add changelog entry.

Use strings for the ApplicationSetting properties:
 - restricted_visibility_levels
 - default_project_visibility
 - default_snippet_visibility
 - default_group_visibility

Because environments also expose the project, ensure the projects are
exposed as they were before in API v3.

Instead of exposing the VisibilityLevel as Integer, expose it as
String `visibility`.

Instead of exposing the VisibilityLevel as Integer, expose it as
String `visibility` for Project and ProjectSnippet.

Filter queries also accept the `visibility` as String instead of
`visibility_level` as Integer.

Also remove the `public` boolean.

Return 204 on all DELETE endpoints

See merge request !1312

Add docs on how to install GitLab on GCP

See merge request !1340

[ci skip]

For the API, the VisibilityLevel will be exposed as String instead of
Integer. So add the string values and method to translate a level
integer to a string.

Correct the return code description for 204

See merge request !9648

CORS: Whitelist pagination headers

Closes #28405

See merge request !9651

Remove jQuery UI

Closes #18437

See merge request !9649

Fix updaing commit status when using optional attributes

Closes #28656 and #25784

See merge request !9618

CE Upstream - Wednesday

See merge request !1330

Use v4 endpoint in API docs

See merge request !1338

API issues - support filtering by iids

Closes #28257

See merge request !9541

cleanup SSH key details

Closes #28896

See merge request !9643

Enable filtering milestones by search criteria in the API

Closes #28807

See merge request !9606

Signed-off-by: Rémy Coutable <remy@rymai.me>

jQuery UI is no longer used anywhere so say goodbye to it from package.json & Gemfile

Merge branch '28805-download-archive-with-branch-like-feature-xxxx-add-extra-directory-level' into 'master'

Ensure archive download is only one directory deep

Closes #28805

See merge request !9616

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>