Add a return type hint to SearchApiElasticsearchBackend::__sleep()

See #3425235-12: Make the Search API module compatible with Drupal 11: In order to make the Search API module compatible with Drupal 11, we need to add an array return type hint to the BackendPluginBase::__sleep() method. To avoid a fatal error, the return type hint has to be added to the overriding method, SearchApiElasticsearchBackend::__sleep(), before the Search API change goes in. (Ideally, you’ll also create a new release before the Search API change is committed, so probably right after committing this issue.)