kubeadm: deprecate the RootlessControlPlane feature gate

The feature was left in alpha for a few releases, since
k8s started planning on a broader feature UserNamespacesSupport,
which is what kubeadm should integrate with.

UserNamespacesSupport graduated to beta in 1.30.
Once it graduates to GA kubeadm can start using it an remove
RootlessControlPlane.

Author: neolit123

cmd/kubeadm/app/features/features.go

@@ -47,7 +47,10 @@ var InitFeatureGates = FeatureList{
 		DeprecationMessage: "The PublicKeysECDSA feature gate is deprecated and will be removed when v1beta3 is removed." +
 			" v1beta4 supports a new option 'ClusterConfiguration.EncryptionAlgorithm'.",
 	},
-	RootlessControlPlane:             {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
+	RootlessControlPlane: {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha},
+		DeprecationMessage: "Deprecated in favor of the core kubelet feature UserNamespacesSupport which is beta since 1.30." +
+			" Once UserNamespacesSupport graduates to GA, kubeadm will start using it and RootlessControlPlane will be removed.",
+	},
 	EtcdLearnerMode:                  {FeatureSpec: featuregate.FeatureSpec{Default: true, PreRelease: featuregate.Beta}},
 	WaitForAllControlPlaneComponents: {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
 }