API: Use `visibility` as string parameter everywhere

Closes #27501

See merge request !9337

Correct the return code description for 204

See merge request !9648

And add changelog entry.

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

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

API issues - support filtering by iids

Closes #28257

See merge request !9541

Enable filtering milestones by search criteria in the API

Closes #28807

See merge request !9606

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.