Test against minimum supported version

Follow up of #1110:
The new minimum supported version is 3.9.0.
Therefore, we should test against that version to make sure the
cluster-operator is compatible with the minimum supported version.

Author: ansd

.github/workflows/pr.yml

@@ -53,7 +53,7 @@ jobs:
         k8s:
         - v1.24.1
         rabbitmq-image:
-        - rabbitmq:3.9-management
+        - rabbitmq:3.9.0-management # minimum supported version
         - rabbitmq:3.10-management
         - pivotalrabbitmq/rabbitmq:master-otp-min
         - pivotalrabbitmq/rabbitmq:master-otp-max