prow.sh: support Kubernetes 1.21

Jobs that specifically use Kubernetes 1.21 have to use the unrelease
KinD because the latest stable release, 0.10.0, is not compatible with
it.

This special case can be removed once the next KinD release is out.

Author: pohly

Diff for: prow.sh

@@ -136,6 +136,9 @@ kind_version_default () {
     case "${CSI_PROW_KUBERNETES_VERSION}" in
         latest|master)
             echo main;;
+        1.21*|release-1.21)
+            # TODO: replace this special case once the next KinD release supports 1.21.
+            echo main;;
         *)
             echo v0.10.0;;
     esac