etcdserver: Fix 64 KB websocket notification message limit

This fixes etcd being unable to send any message longer than 64 KB as
a notification over the websocket. This was because the older version
of grpc-websocket-proxy was used and WithMaxRespBodyBufferSize option
wasn't set.

embed/serve.go

@@ -286,6 +286,7 @@ func (sctx *serveCtx) createMux(gwmux *gw.ServeMux, handler http.Handler) *http.
 						return outgoing
 					},
 				),
+				wsproxy.WithMaxRespBodyBufferSize(0x7fffffff),
 			),
 		)
 	}