Add support for Elasticsearch version 7.6.*

- Upgrade elasticsearch-rest-client to 7.6.0.
- Add V7_6 to the version manager, and use this in all the V7 tests.
- Use the 7.6.0 image from blacktop in the test container; based on [1].
- Triggered by watching releases [2], which links to the release notes.

[1] https://github.com/blacktop/docker-elasticsearch-alpine/pull/27
[2] https://github.com/elastic/elasticsearch/releases/tag/v7.6.0

Change-Id: I63e6c396b8c159b4ebdfe9817518f239fd62928c

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

@@ -64,6 +64,8 @@ public class ElasticContainer extends ElasticsearchContainer {
         return "blacktop/elasticsearch:7.4.2";
       case V7_5:
         return "blacktop/elasticsearch:7.5.2";
+      case V7_6:
+        return "blacktop/elasticsearch:7.6.0";
     }
     throw new IllegalStateException("No tests for version: " + version.name());
   }