Migrate ghost records migrate audit events to Auditor

What does this MR do and why?

This MR refactors audit events in ee/app/services/ee/users/migrate_records_to_ghost_user_service.rb from using the deprecated AuditEventService.new to the modern ::Gitlab::Audit::Auditor.audit pattern.

What this MR changes:

• Replaces AuditEventService.new with ::Gitlab::Audit::Auditor.audit in the log_audit_event method
• Creates a new audit event type configuration user_records_migrated_to_ghost.yml
• Updates the corresponding test expectations to match the new audit pattern
• Maintains backward compatibility while modernizing the audit event implementation

Why this change is needed:

• Part of the broader initiative to standardize all audit events across GitLab to use the Auditor pattern
• Improves audit event consistency and maintainability
• Enables better streaming and processing of audit events
• Follows the established patterns for modern audit event implementation

Related Issues

Closes #567635 (closed)

Related to #556329 (closed) - "Ensure all audit events are using Auditor"

Steps to Verify

1. Set up a test environment:
   • Create a test user account with some content (issues, merge requests, comments, etc.)
   • Ensure you have admin privileges to delete users
2. Delete the user:
   • Go to Admin Area → Users
   • Find the test user and click on their profile
   • Click Delete user
   • Choose to delete the user (not hard delete) so the migration service runs
3. Verify audit events are generated:
   • Navigate to Admin Area → Monitoring → Audit Events (instance-level)
   • Look for audit events with the user_records_migrated_to_ghost event type
   • The event should show details about the user being migrated to ghost user
4. Check the audit event details: The audit event should contain:
   • Event type: user_records_migrated_to_ghost
   • Target: The deleted user
   • Author: The admin who performed the deletion
   • Message indicating the user records were migrated to ghost user
   • Additional details including the migration action

Screenshots/Recordings