Qualify external-attacher 2.1.1 in RC staging

Author: verult

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

@@ -11,7 +11,7 @@ images:
   newTag: "v1.5.0-gke.0"
 - name: gke.gcr.io/csi-attacher
   newName: gcr.io/gke-release-staging/csi-attacher
-  newTag: "v2.1.0-gke.0"
+  newTag: "v2.1.1-gke.0"
 - name: gke.gcr.io/csi-node-driver-registrar
   newName: gcr.io/gke-release-staging/csi-node-driver-registrar
   newTag: "v1.2.0-gke.0"