In API V4 all endpoints were changed so Merge Requests and Issues
should be referred by iid, instead of id. Except the /notes endpoint
was forgotten. So change the endpoints from:

- /projects/:id/issues/:issue_id/notes
- /projects/:id/merge_requests/:merge_request_id/notes

To:

- /projects/:id/issues/:issue_iid/notes
- /projects/:id/merge_requests/:merge_request_iid/notes

For Project Snippets nothing changes.