V4 API: Remove deprecated MR and Issue endpoints and preserve V3 namespace

Keep V3 API untouched while making the following changes for V4:

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

EE Backport: gitlab-ee!1166

---

• 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)