Add audit event name for `merge method` in Project

What does this MR do and why?

Adds a meaningful audit event name for merge method change in Project settings

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
   1. Hint: use https://webhook.site/
2. Create a new project https://docs.gitlab.com/ee/user/project/working_with_projects.html#create-a-project
3. Change the settings for Merge method in Project Settings https://docs.gitlab.com/ee/user/project/settings/
4. Check audit event is streamed with correct event name.

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 #369288 (closed)