API routes referencing a specific issue should use the issue `iid`

What does this MR do?

• Use Issues and MR iids instead of id for API v4 routes of the form /projects/:project_id/issues/:issue_id

Are there points in the code the reviewer needs to double check?

• Make sure there aren't any endpoints that need to be migrated to IID that aren't covered by this MR.
• Make sure v3 compatibility is maintained.

Why was this MR needed?

• This change makes the API more closely resemble the routes of the web interface.
• Since this is a breaking change, this should go into API v4.

Tasks

• Implementation
  • Modify routes in lib/api/issues
  • Modify routes in lib/api/merge_requests
  • Any other instances of :merge_request_id
    • Merge request diffs
    • Award emoji
    • Todos API
    • TimeTrackingEndpoints
  • Any other instances of :issue_id
    • Award emoji
    • Todos API
    • TimeTrackingEndpoints
• Tests
  • Added
  • Passing
• Meta
  • CHANGELOG entry created
  • Branch has no merge conflicts with master
  • Squashed related commits together
  • Added screenshots
  • Check for clean merge with EE
  • Documentation added/updated
    • Merge requests API
    • Issues API
    • Award emoji API
    • Todos API
    • Time tracking endpoints
    • Update v3-to-v4 document
• Follow-up
  • Sanity check from command line
  • Verify that V3 API is unaffected
    • Backport "time tracking endpoints" to v3
    • Any other V3 backports to be made?
  • Verify that award emoji conflict was resolved correctly
  • Verify that conflicts on EE branch were resolved correctly
• Review
  • Reviewer
  • Maintainer
• Wait for merge

• Closes #28251 (closed)
• Related EE MR: gitlab-org/gitlab-ee!1325