API: Use Issues and MRs IID instead ID for project scope API query

In order to stay consistent with the web interface, we're planning to use Issues and MRs :iid instead :id for /projects/:id/issues/:id and /projects/:id/merge_requests/:id endpoints (and endpoints under these like /projects/:id/merge_requests/:id/merge).

Further discussion can be found on #21699 (closed)

Important

This change should not affect V3 API.