text: Title: [Serve] Display error message on old service before update mode is introduced.
  Body: <!-- Describe the bug report / feature request here -->
  
  Seems like in the following code, we just silently ignored the `mode` argument.
  
  https://github.com/skypilot-org/skypilot/blob/d5b6d89c83ea1ee7258f68314da4c6f8add83e04/sky/serve/serve_utils.py#L952-L966
  
  Expected behaviour: if a user specified a rolling update on an old service, we should raise an error says that rolling update is not supported on this service.
  