Migrate existing RabbitMQ classic mirrored queues to quorum queues before upgrading to RabbitMQ 4.x, where classic mirroring is removed

Steps

1. Know that classic queue mirroring (ha-mode/ha-policy) was deprecated in RabbitMQ 3.9 and fully removed in 4.0 — classic queues become single-replica only, and policies with ha-mode keys are silently ineffective (or rejected if newly created) on 4.x
2. On the pre-4.x ('blue') cluster, export definitions with rabbitmqadmin v2 using the migration transforms, e.g. `rabbitmqadmin --host blue definitions export --file blue.json -t prepare_for_quorum_queue_migration,drop_empty_policies`
3. Review the transformed definitions file — queues carrying mirroring policies are rewritten to declare x-queue-type=quorum, and policies left empty after stripping ha-mode keys are dropped
4. Stand up a new RabbitMQ 4.x cluster ('green') sized for at least 3 nodes, since quorum queues require a minimum of 3 replicas
5. Import the transformed definitions into the green cluster and cut traffic over (blue/green migration; there is no in-place classic-to-quorum conversion command)
6. Confirm no policy on the new cluster references ha-mode, ha-params, or ha-sync-mode, since RabbitMQ 4.x rejects newly created policies containing those keys