Log user approval/rejection in application logs

Proposal

As part of &4491 and &4315 (closed), we should track when a member has been approved or rejected by an admin in application logs.

Relates to: #276250 (closed) which logs an audit event on user registration approval. Also relates to: #298783 (closed), which logs an audit event on user instance access request and request rejection.

MR: !51768 (merged)