Reinstate legacy taint for control plane nodes

Similar to APIs, we don't delete legacy taints and labels from the page,
we leave them there and mark them as deprecated.

Reinstate taint node-role.kubernetes.io/master.

Author: Tim Bannister

content/en/docs/reference/labels-annotations-taints/_index.md

@@ -738,10 +738,20 @@ Used on: Node
 
 Label that kubeadm applies on the control plane nodes that it manages.
 
-### node-role.kubernetes.io/control-plane
+### node-role.kubernetes.io/control-plane {#node-role-kubernetes-io-control-plane-taint}
 
 Used on: Node
 
 Example: `node-role.kubernetes.io/control-plane:NoSchedule`
 
 Taint that kubeadm applies on control plane nodes to allow only critical workloads to schedule on them.
+
+### node-role.kubernetes.io/master (deprecated) {#node-role-kubernetes-io-master-taint}
+
+Used on: Node
+
+Example: `node-role.kubernetes.io/master:NoSchedule`
+
+Taint that kubeadm previously applied on control plane nodes to allow only critical workloads to schedule on them.
+Replaced by [`node-role.kubernetes.io/control-plane`](#node-role-kubernetes-io-control-plane-taint); kubeadm
+no longer sets or uses this deprecated taint.