Remove references to etcd2 in v1.13 since support has been removed (#…

…11414)

* Remove etcd2 references as etcd2 is deprecated

Link back to the v1.12 version of the etcd3 doc for
the etcd2->etcd3 migration instructions.

I updated the kube-apiserver reference manually,
unsure if that is auto-generated somehow.

The federation-apiserver can still potentially
support etcd2 so I didn't touch that.

* Remove outdated {master,node}.yaml files

There are master/node yaml files that reference
etcd2.service that are likely highly out of date.
I couldn't find any docs that actually reference
these templates so I removed them

* Address review comments

content/en/docs/reference/command-line-tools-reference/kube-apiserver.md

@@ -857,7 +857,7 @@ kube-apiserver [flags]
       <td colspan="2">--storage-backend string</td>
     </tr>
     <tr>
-      <td></td><td style="line-height: 130%; word-wrap: break-word;">The storage backend for persistence. Options: 'etcd3' (default), 'etcd2'.</td>
+      <td></td><td style="line-height: 130%; word-wrap: break-word;">The storage backend for persistence. Options: 'etcd3' (default)</td>
     </tr>
 
     <tr>

content/en/docs/reference/using-api/api-concepts.md

@@ -86,7 +86,7 @@ For example:
         }
         ...
 
-A given Kubernetes server will only preserve a historical list of changes for a limited time. Older clusters using etcd2 preserve a maximum of 1000 changes. Newer clusters using etcd3 preserve changes in the last 5 minutes by default.  When the requested watch operations fail because the historical version of that resource is not available, clients must handle the case by recognizing the status code `410 Gone`, clearing their local cache, performing a list operation, and starting the watch from the `resourceVersion` returned by that new list operation. Most client libraries offer some form of standard tool for this logic. (In Go this is called a `Reflector` and is located in the `k8s.io/client-go/cache` package.)
+A given Kubernetes server will only preserve a historical list of changes for a limited time. Clusters using etcd3 preserve changes in the last 5 minutes by default.  When the requested watch operations fail because the historical version of that resource is not available, clients must handle the case by recognizing the status code `410 Gone`, clearing their local cache, performing a list operation, and starting the watch from the `resourceVersion` returned by that new list operation. Most client libraries offer some form of standard tool for this logic. (In Go this is called a `Reflector` and is located in the `k8s.io/client-go/cache` package.)
 
 ## Retrieving large results sets in chunks