feat(mgmt): auto-add rp prefix to Severity (#5112)

`Severity` as introduced in common types V5: https://github.com/Azure/azure-rest-api-specs/blob/main/specification/common-types/resource-management/v5/networksecurityperimeter.json#L173
We should auto add rp refix to it.

resolve #5039

src/AutoRest.CSharp/Common/Input/InputTypes/Transformation/InputCommonSingleWordModelTransformer.cs

@@ -32,7 +32,8 @@ internal class InputCommonSingleWordModelTransformer
             "PrivateLinkServiceConnectionStateProperty",
             // internal, but could be public in the future, also make the names more consistent
             "PrivateEndpointConnectionListResult",
-            "PrivateLinkResourceListResult"
+            "PrivateLinkResourceListResult",
+            "Severity"
         };
 
         public static void Transform(InputNamespace inputNamespace)