Document Supported CNI Plugins in OSSM Pre-reqs

We should document which CNI (Networking) Plugins are supported with Service Mesh.

This can likely be added to the pre-reqs section: https://docs.openshift.com/container-platform/4.6/service_mesh/v2x/preparing-ossm-installation.html#prerequisites

 In OCP 4.7 / OSSM 2.0, we document OVNKubernetes as a tech preview support:

https://docs.openshift.com/container-platform/4.7/service_mesh/v2x/servicemesh-release-notes.html#ovnkubernetes-technology-preview

We have now completed QE on OVN, and should be in a position to declare full support: https://issues.redhat.com/browse/OSSM-282.

Thus, we should document that OSSM 2.0 is fully supported on:

• Openshift-SDN (OCP 4.6, 4.7)
• OVN-Kubernetes (Service Mesh 2.0.1+ on OCP 4.6.32+ and OCP 4.7.14+)

In the future, we will also look to support service mesh on 3rd party SDNs that have passed our conformance tests. We do not need to address this in our docs just yet though.