Add support for Elasticsearch version 7.9.*

Upgrade elasticsearch-rest-client to 7.9.3, which is the currently
latest 7.9 minor version available.

Add V7_9 to the version manager and use it for all the V7 tests.

Change-Id: I7d76ece60d3ed7400bcf28bcd45d0e2e02870ccd

javatests/com/google/gerrit/elasticsearch/ElasticContainer.java

@@ -53,6 +53,8 @@ public class ElasticContainer extends ElasticsearchContainer {
         return "blacktop/elasticsearch:7.7.1";
       case V7_8:
         return "blacktop/elasticsearch:7.8.1";
+      case V7_9:
+        return "blacktop/elasticsearch:7.9.3";
     }
     throw new IllegalStateException("No tests for version: " + version.name());
   }