Resolve "Store MergeWorker JID on merge request, and clean up stuck merges"

What does this MR do?

1. Saves merge_jid on merge_requests
2. Removes unnecessary locked_at column from merge_requests
3. Create clean-up worker to update the state of stuck MR's
4. Add clean-up worker to Cron
5. Stop using Locked state, use Merging instead

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

MergeRequest#locked_at column being removed

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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 #31207 (closed)