cluster-rke2-migrate-legacy-etcd-certs.sh: add safeguard to migrate_cluster

Closes #1870 (closed)

This adds a check to cluster-rke2-migrate-legacy-etcd-certs.sh, to ensure that in the (very unlikely) case where a cluster would not yet have the cluster.x-k8s.io/cluster-name we fail with a clear error instead of silently calling migrate_cluster with wrong parameters.