Event type information for group merge request setting updates

What does this MR do and why?

Adds a meaningful name for the following events and actions that can be used to differentiate them from other events:

Event	Action	Location
allow_author_approval	Update	file
allow_committer_approval	Update	file
allow_overrides_to_approver_list_per_merge_request	Update	file
retain_approvals_on_push	Update	file
require_password_to_approve	Update	file

How to set up and validate locally

1. Setup audit event streaming on a group https://docs.gitlab.com/ee/administration/audit_event_streaming.html#add-a-new-streaming-destination
2. Change Group merge request settings. Its under Group -> Settings -> General -> Merge request approvals
3. Check audit event is streamed with correct event name.

Note: First time when you change setting it will stream event name for creation because first time changing settings creates GroupMergeRequestSetting and after that you can try changing again to see update events.

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 #373949 (closed)