Add audit events for fork unlink action

What does this MR do and why?

This MR intends to add a project level audit event when the user removes the fork relationship.

• Related issue: #272532 (closed)

Adds a new EE scoped service UnlinkForkService that overrides the execute method in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/projects/unlink_fork_service.rb service - This adds a project level audit event for fork unlink action.

• Referred docs: https://docs.gitlab.com/ee/development/audit_event_guide/

Screenshots or screen recordings

After the removal of fork relationship::

How to set up and validate locally

1. Checkout to this feature branch
2. Fork a project
3. In the forked project, unlink the fork relationship from Settings -> General -> Advanced section click Remove fork relationship button
4. Navigate to Security & Compliance -> Audit events section
5. Notice that the fork unlink action is logged in the audit events list

NoteL Audit events feature is only available for paid tiers.

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.