WIP: Freeze API V3

What does this MR do?

It freezes the API V3 code.

Are there points in the code the reviewer needs to double check?

Nothing that was changed for V3 already is not reverted.

Why was this MR needed?

To avoid V4 changes breaks something in V3.

File checklist

• modified lib/api/api.rb
• new file lib/api/v3/access_requests.rb
• new file lib/api/v3/api.rb
• new file lib/api/v3/api_guard.rb
• modified lib/api/v3/boards.rb
• modified lib/api/v3/branches.rb
• modified lib/api/v3/broadcast_messages.rb
• modified lib/api/v3/builds.rb
• new file lib/api/v3/commit_statuses.rb
• modified lib/api/v3/commits.rb
• modified lib/api/v3/deploy_keys.rb
• modified lib/api/v3/deployments.rb
• modified lib/api/v3/entities.rb
• modified lib/api/v3/environments.rb
• new file lib/api/v3/events.rb
• new file lib/api/v3/features.rb
• modified lib/api/v3/files.rb
• new file lib/api/v3/group_milestones.rb
• new file lib/api/v3/group_variables.rb
• modified lib/api/v3/helpers.rb
• new file lib/api/v3/internal.rb
• modified lib/api/v3/issues.rb
• new file lib/api/v3/jobs.rb
• new file lib/api/v3/keys.rb
• modified lib/api/v3/labels.rb
• new file lib/api/v3/lint.rb
• modified lib/api/v3/members.rb
• new file lib/api/v3/milestone_responses.rb
• modified lib/api/v3/milestones.rb
• new file lib/api/v3/namespaces.rb
• modified lib/api/v3/notes.rb
• new file lib/api/v3/notification_settings.rb
• new file lib/api/v3/pagination_params.rb
• new file lib/api/v3/pipeline_schedules.rb
• modified lib/api/v3/pipelines.rb
• modified lib/api/v3/project_hooks.rb
• new file lib/api/v3/project_milestones.rb
• modified lib/api/v3/project_snippets.rb
• modified lib/api/v3/projects.rb
• new file lib/api/v3/protected_branches.rb
• modified lib/api/v3/repositories.rb
• new file lib/api/v3/runner.rb
• modified lib/api/v3/runners.rb
• new file lib/api/v3/scope.rb
• modified lib/api/v3/services.rb
• new file lib/api/v3/session.rb
• modified lib/api/v3/settings.rb
• new file lib/api/v3/sidekiq_metrics.rb
• modified lib/api/v3/snippets.rb
• modified lib/api/v3/subscriptions.rb
• modified lib/api/v3/system_hooks.rb
• modified lib/api/v3/tags.rb
• modified lib/api/v3/time_tracking_endpoints.rb
• modified lib/api/v3/todos.rb
• modified lib/api/v3/triggers.rb
• modified lib/api/v3/users.rb
• modified lib/api/v3/variables.rb
• new file lib/api/v3/version.rb

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #35430 (moved).