Remove immutable annotation from engine_version to

support version upgrade.

Author: singh-shreya321

mmv1/products/memorystore/Instance.yaml

@@ -257,8 +257,7 @@ properties:
             default_from_api: true
   - name: 'engineVersion'
     type: String
-    description: "Optional. Immutable. Engine version of the instance. "
-    immutable: true
+    description: "Optional. Engine version of the instance."
     default_from_api: true
   - name: 'engineConfigs'
     type: KeyValuePairs