Add feature flag to stop legacy audit_events table writes

What does this MR do and why?

Add feature flag to stop legacy audit_events table writes

Introduce the stop_legacy_audit_event_writes feature flag to control dual-writes to the legacy audit_events table. When enabled, audit events are written only to the new scoped tables (project_audit_events, group_audit_events, instance_audit_events, user_audit_events).

This allows gradual rollout and instant rollback of the migration away from the legacy table. The new tables use a shared sequence for ID generation, ensuring uniqueness across all audit event tables.

Changelog: added

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