Add API endpoint to get all milestone merge requests

What does this MR do?

It adds a new API endpoint to retrieve all merge request related to a single project milestone.

Why was this MR needed?

It makes it a lot easier to get all milestone merge request and fixes an existing open issue. It also makes it consistent with fetching all milestone issues.

Does this MR meet the acceptance criteria?

• 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 #22018 (closed)