deprecate RBD plugin from available in-tree drivers

Based on https://groups.google.com/g/kubernetes-sig-storage/c/h5751_B5LQM, the
consensus was to start the deprecation in v1.28.

This commit start the deprecation process of RBD plugin from in-tree
drivers.

ACTION REQUIRED:
   RBD volume plugin ( `kubernetes.io/rbd`) has been deprecated in this release
   and will be removed in a subsequent release. Alternative is to use RBD CSI driver
   (https://github.com/ceph/ceph-csi/) in your Kubernetes Cluster.

Signed-off-by: Humble Chirammal <humble.devassy@gmail.com>

pkg/api/persistentvolume/util_test.go

@@ -212,6 +212,27 @@ func TestWarnings(t *testing.T) {
 				`spec.persistentVolumeSource.photonPersistentDisk: deprecated in v1.11, non-functional in v1.16+`,
 			},
 		},
+		{
+			name: "PV RBD deprecation warning",
+			template: &api.PersistentVolume{
+				Spec: api.PersistentVolumeSpec{
+					PersistentVolumeSource: api.PersistentVolumeSource{
+						RBD: &api.RBDPersistentVolumeSource{
+							CephMonitors: nil,
+							RBDImage:     "",
+							FSType:       "",
+							RBDPool:      "",
+							RadosUser:    "",
+							Keyring:      "",
+							SecretRef:    nil,
+							ReadOnly:     false,
+						},
+					},
+				},
+			},
+			expected: []string{
+				`spec.rbd: deprecated in v1.28, non-functional in v1.31+`},
+		},
 		{
 			name: "PV ScaleIO deprecation warning",
 			template: &api.PersistentVolume{