Add support for Elasticsearch version 7.7.*

Upgrade elasticsearch-rest-client to 7.7.0.

Add V7_7 to the version manager, and use this in all the V7 tests.

Change-Id: I406ccf0bc29f38ccce8ab74f5742342bc50251d2

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

@@ -58,6 +58,8 @@ public class ElasticContainer extends ElasticsearchContainer {
         return "blacktop/elasticsearch:7.5.2";
       case V7_6:
         return "blacktop/elasticsearch:7.6.2";
+      case V7_7:
+        return "blacktop/elasticsearch:7.7.0";
     }
     throw new IllegalStateException("No tests for version: " + version.name());
   }