add non upgrade channel (#11492)

Co-authored-by: Xiahe Liu <Xiahe.Liu@microsoft.com>

Author: xhl873

specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2020-11-01/managedClusters.json

@@ -2786,7 +2786,8 @@
           "enum": [
             "rapid",
             "stable",
-            "patch"
+            "patch",
+            "none"
           ],
           "x-ms-enum": {
             "name": "upgradeChannel",