Fix deprecation warning about missing return type hint for ConfigEventsSubscriber::getSubscribedEvents()

With recent versions of Drupal, you get the following deprecation warning:

Method "Symfony\Component\EventDispatcher\EventSubscriberInterface::getSubscribedEvents()" might add "array" as a native return type declaration in the future. Do the same in implementation "Drupal\search_api_attachments\EventSubscriber\ConfigEventsSubscriber" now to avoid errors or add an explicit @return annotation to suppress this message.