Add back supported platforms for OVN-Kubernetes migration

Somehow this entire section went missing between 4.7 and 4.8.
This adds the section back along with RHV as a platform.

- https://issues.redhat.com/browse/OSDOCS-2304

Author: jboxman

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

@@ -7,6 +7,17 @@
 
 Migrating to the OVN-Kubernetes Container Network Interface (CNI) cluster network provider is a manual process that includes some downtime during which your cluster is unreachable. Although a rollback procedure is provided, the migration is intended to be a one-way process.
 
+
+A migration to the OVN-Kubernetes cluster network provider is supported on the following platforms:
+
+* Bare metal hardware
+* Amazon Web Services (AWS)
+* Google Cloud Platform (GCP)
+* Microsoft Azure
+* {rh-openstack-first}
+* {rh-virtualization-first}
+* VMware vSphere
+
 [id="considerations-migrating-ovn-kubernetes-network-provider_{context}"]
 == Considerations for migrating to the OVN-Kubernetes network provider