Move /projects/fork/:id to /projects/:id/fork in V4 API

Simply move /projects/fork/:id to /projects/:id/fork as part of V4 breaking change.

• 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 #14492 (closed)