Add audit events for merge request settings

See merge request gitlab-org/gitlab!83922

(cherry picked from commit 8c6f4a33)

859d2a8b Add audit events for merge request settings
e6f7a15c Apply 10 suggestion(s) to 5 file(s)
7b03a8e9 Disable audit on column change from nil to false/blank
3207a415 Move blank to blank logic in auditor