Elasticsearch Connector -> Search Api Integration -> Implement search_api_grouping

Elasticsearch 8 does support field collapsing and field denormalization; so it's still worth doing this.

However, the Search API Grouping module → is only compatible with Drupal core 8 and 9 (both of which are unsupported); and it (currently) has a hard dependency on search_api_solr. So I'll move this to the 8.0.x branch, but mark it as postponed for now.