Add logging for approval deletions on merged/locked MRs

Marc Shawrequested to merge
duo-edit-20251224-103432 into master

What does this MR do?

Adds observational logging when approval deletion is attempted on merged or locked (merge in progress) merge requests.

Why?

Related to #557028 - investigating cases where approvals go missing on merged MRs.

Feature flag

log_merged_mr_approval_deletion - disabled by default

Relates to #557028

Merge request reports