Feature flag rollout: log_merged_mr_approval_deletion
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=584829)
</details>
<!--IssueSummary end-->
## Summary
This issue tracks the rollout of the `log_merged_mr_approval_deletion` feature flag introduced in !217440.
This feature flag enables observational logging when approval deletion is attempted on merged or locked (merge in progress) merge requests. This is part of the investigation for #557028 where approvals are going missing on merged MRs.
## Owners
- Team: Code Review
- Slack channel: `#g_code_review`
- Feature flag issue owner: @marc_shaw
## The Rollout Plan
### Rollout Steps
- [ ] Enable on staging
- [ ] Test on staging
- [ ] Ensure that logging is working correctly in Kibana
- [ ] Enable on GitLab.com for a subset of projects/groups
- [ ] Enable on GitLab.com globally
- [ ] Monitor logs for any approval deletion attempts on merged/locked MRs
- [ ] Analyze collected data to understand the root cause of #557028
### Rollback Steps
- [ ] Disable the feature flag
## What does the feature flag control?
When enabled, this feature flag logs a warning message whenever approval deletion is attempted on a merged or locked merge request. The logging includes:
- Source of the deletion attempt
- Cause (if provided)
- Merge request ID and IID
- Merge request state
- Project ID
- Current user ID
The logging is added in the following locations:
- `MergeRequests::RemoveApprovalService`
- `EE::MergeRequests::BaseService#delete_approvals`
- `MergeRequests::ResetApprovalsService#perform_code_owner_approval_deletion`
- `API::MergeRequestApprovals#reset_approvals`
## Feature flag type
This is a **development** feature flag used for observational/debugging purposes.
## Related Links
- Introducing MR: !217440
- Related issue: #557028
issue