Remove rkt

https://github.com/rkt/rkt is an ex-project.
rkt has kicked the bucket, shuffled off its mortal coil, run down the
curtain and joined the choir invisible.

Author: Tim Bannister

content/en/docs/reference/glossary/rkt.md

@@ -72,7 +72,7 @@ <h2 style="color: #3771e3;">Cluster Diagram</h2>
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                 <p><b>The Master is responsible for managing the cluster.</b> The master coordinates all activities in your cluster, such as scheduling applications, maintaining applications' desired state, scaling applications, and rolling out new updates.</p>
-                <p><b>A node is a VM or a physical computer that serves as a worker machine in a Kubernetes cluster.</b> Each node has a Kubelet, which is an agent for managing the node and communicating with the Kubernetes master. The node should also have tools for handling container operations, such as Docker or rkt. A Kubernetes cluster that handles production traffic should have a minimum of three nodes.</p>
+                <p><b>A node is a VM or a physical computer that serves as a worker machine in a Kubernetes cluster.</b> Each node has a Kubelet, which is an agent for managing the node and communicating with the Kubernetes master. The node should also have tools for handling container operations, such as containerd or Docker. A Kubernetes cluster that handles production traffic should have a minimum of three nodes.</p>
 
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