7.0.0b: add rabbitmq migration (#1237)

Signed-off-by: Christian Berendt <berendt@osism.tech>

doc/source/notes/7.0.0b.rst

@@ -25,6 +25,13 @@ First things first
   the Keycloak service, do not upgrade the Keycloak service and contact us in
   advance.
 
+* The switch from classic queue mirroring and durable queues to quorum queues
+  in RabbitMQ has not yet been tested and documented. So far, there is only the
+  `Kolla-Ansible documentation <https://docs.openstack.org/kolla-ansible/latest/reference/message-queues/rabbitmq.html#high-availability
+>`_,
+  which requires all services to be stopped. We are still working on a better
+  approach.
+
 General notes
 =============