audit merged MRs

What does this MR do and why?

• Refs: #442279 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

mr_audit_events

How to set up and validate locally

1. Enable the FF in bundle exec rails console: Feature.enable :ff_compliance_audit_mr_merge
2. Merge something
3. Visit audit events