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

java/com/google/gerrit/elasticsearch/ElasticVersion.java

@@ -27,7 +27,8 @@ public enum ElasticVersion {
   V7_3("7.3.*"),
   V7_4("7.4.*"),
   V7_5("7.5.*"),
-  V7_6("7.6.*");
+  V7_6("7.6.*"),
+  V7_7("7.7.*");
 
   private final String version;
   private final Pattern pattern;