Add 0.6.0-rc1 to staging rc test grid

Author: davidz627

deploy/kubernetes/overlays/prow-gke-release-staging-rc/kustomization.yaml

@@ -5,7 +5,7 @@ bases:
 images:
 - name: gke.gcr.io/gcp-compute-persistent-disk-csi-driver
   newName: gcr.io/gke-release-staging/gcp-compute-persistent-disk-csi-driver
-  newTag: "v0.5.1-rc1"
+  newTag: "v0.6.0-rc1"
 - name: gke.gcr.io/csi-provisioner
   newName: gcr.io/gke-release-staging/csi-provisioner
   newTag: "v1.2.0-gke.0"