OSDOCS-5686: Added a note about OVN-Kubernetes migration

Author: EricPonvelle

modules/nw-ovn-kubernetes-migration-about.adoc

@@ -18,6 +18,11 @@ A migration to the OVN-Kubernetes network plugin is supported on the following p
 * {rh-virtualization-first}
 * VMware vSphere
 
+[IMPORTANT]
+====
+Migrating to or from the OVN-Kubernetes network plugin is not supported for managed OpenShift cloud services such as OpenShift Dedicated and Red Hat OpenShift Service on AWS (ROSA).
+====
+
 [id="considerations-migrating-ovn-kubernetes-network-provider_{context}"]
 == Considerations for migrating to the OVN-Kubernetes network plugin