Audit event for group level merge request settings

What does this MR do and why?

Adds audit events for merge request approval settings for Groups.

Screenshots

Approval settings creation:

Approval settings update:

How to set up and validate locally

1. Create new group.
2. Go to Group > Settings > General and expand Merge request approvals.
3. Enable some checkboxes and hit Save changes.
4. Check audit events at {{base_url}}/admin/audit_logs.
5. Repeat step 2 to 4 and check update works as expected.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356152 (closed)