Enable "v4" API namespace

It enables the v4 namespace for our API. Doing this, every call to a /v4 endpoint shall behave the same as /v3, unless we change this behavior on v4.

It's covering #25183 (closed) as a way to discuss the best approach for growing v4 changes without affecting existing v3 endpoints.

API changes

• Remove iid filter on projects/x/issues
• Rename iid filter on projects/x/issues to iids
• Remove projects/x/merge_request/y endpoint

---

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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 #27180 (closed), #25183 (closed)