Remove limitation in docs

Description of problem per Marc Curry and Peng Liu in Slack discussion:

 
I find one place we need to change. In 4.12, as we allow users to specify the v4InternalSubnet and v6InternalSubnet . So the following limitation can be removed.
If your cluster uses any part of the 100.64.0.0/16 IP address range, you cannot migrate to the OVN-Kubernetes network plugin because it uses this IP address range internally.
Instead, we could say
If your cluster or surrounding network uses any part of the 100.64.0.0/16 address range, you need to choose another unused ip range for ovn-kubernetes by specifying the v4InternalSubnet under `spec.defaultNetwork.ovnKubernetesConfig`. As the ip range 100.64.0.0/16 will be used by ovn-kubernetes internally by default.