MR approval change audit event
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### Proposal
We want a way to capture audit events when a user changes the number of approvers for an MR however:
1. The audit events are captured when the number of approvers is made from the project settings page by navigating to "Settings -\> Merge requests -\> Merge request approvals"
2. The audit events are **not** captured when the number of approvers is made from the MR settings for a specific MR by navigating to "MR# -\> Edit MR -\> Approval Rules"
We would like to capture audit events for the second scenario and be the first to know when developers edit MRs to sidestep approval rules. ideally, this would also want to be available via the API endpoint and streaming.
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