`AuthenticationGenerator` doesn't handle `skip_action_cable` option
so far(`AppGenerator` only handles it).
https://github.com/rails/rails/blob/e6429269fd5a8a7d728557f4e7d7f82c0ad1478c/railties/lib/rails/generators/rails/authentication/authentication_generator.rb#L6-L8

So the current check doesn't work correctly.

Instead of depending on that option, this PR changes to check whether
the engine is enabled or not. We already do the same thing.
https://github.com/rails/rails/blob/e6429269fd5a8a7d728557f4e7d7f82c0ad1478c/railties/lib/rails/commands/app/update_command.rb#L71