Clean merge_jid whenever necessary on the merge process

MergeRequest#merge_jid should be cleaned up whenever we hit a known error on MergeService#execute (or we finished processing). This way we can keep track if the MR is really "ongoing" or just "stuck" (stuck MRs are already being handled by StuckMergeJobsWorker).

Currently, if a an user clicks Merge and the MR is not mergeable, or no source is found, we keep stuck on merge ongoing state.

Backport https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3027

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the [merge request performance guides](https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38476 (closed)