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/acceptance/ssh/ElasticIndexIT.java

@@ -31,7 +31,7 @@ public class ElasticIndexIT extends AbstractIndexTests {
 
   @ConfigSuite.Config
   public static Config elasticsearchV7() {
-    return getConfig(ElasticVersion.V7_6);
+    return getConfig(ElasticVersion.V7_7);
   }
 
   @Override