Add audit event for auto_duo_code_review_enabled changes

What does this MR do and why?

Add audit event for auto_duo_code_review_enabled changes

Add auto_duo_code_review_enabled_updated audit event type to track when the Duo automatic code review setting is changed on a group. This ensures group owners have an audit trail when code review automation settings are modified.

• Create audit event type YAML definition
• Register column in NamespaceSettingChangesAuditor
• Add spec coverage for the new audited setting

References

• Refs: https://gitlab.com/gitlab-org/gitlab/-/work_items/593018

Screenshots or screen recordings

How to set up and validate locally

1. Start a GDK instance with EE license features enabled
2. As a group owner, navigate to a group's settings -> General -> Merge Requests
3. Toggle settings for automatic code reveiw thru Duo
4. Check the audit events log for the group (Group > Security & Compliance > Audit Events)
5. Verify an audit event was created with:
   • Correct event type name auto_duo_code_review_enabled_updated
   • from and to values reflecting the change (e.g., false -> true)
   • target_details showing the group's full path
   • (Or check rails console AuditEvent.last)
6. Verify the event is also streamed if external audit event streaming is configured

MR acceptance checklist

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