ClusterClass rebase should check variable compatibility

PR #8153 removes the variable compatibility checks for Clusters using a ClusterClass when that class is being updated in place. This change is a consequence of external variables.

During ClusterClass rebase - moving a Cluster from one ClusterClass to another - similar variable checks could be done allowing users to safely update from one set of variables to another while changing ClusterClass. Currently validation of those variables is not done during ClusterClass rebase.

This validation should be added to give rebase webhook based validation of variable changes.

/area topology /kind feature