Present enqueued merge jobs as "Merging" as well (https://gitlab.com/gitlab-org/gitlab-ce/issues/31207 follow-up)

What does this MR do?

Tiny addition and follow-up to https://gitlab.com/gitlab-org/gitlab-ce/issues/31207.

It saves the merge_jid right after enqueuing the MergeWorker job, and cleans it up in the end of the merge process. This allows us to present MRs with enqueued jobs as 'Merging' as well, using the MergeRequest#merge_ongoing? method.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Squashed related commits together

What are the relevant issue numbers?

Closes #36114 (closed)