add x-ms-enum to servicefabric (#17508)

Author: ArcturusZhang

specification/servicefabric/resource-manager/Microsoft.ServiceFabric/stable/2021-06-01/cluster.json

@@ -1969,6 +1969,10 @@
         "Windows",
         "Linux"
       ],
+      "x-ms-enum": {
+        "name": "ClusterVersionsEnvironment",
+        "modelAsString": true
+      },
       "x-ms-parameter-location": "method"
     },
     "locationForClusterCodeVersions": {