ElasticContainer: Upgrade V7_8 to elasticsearch 7.8.1

Upgrade elasticsearch-rest-client to 7.8.1, accordingly.

Change-Id: I0c904bc9adaef56e01a2f584e477a6acff959cdd
diff --git a/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java b/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
index 031e255..a51cda4 100644
--- a/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
+++ b/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
@@ -61,7 +61,7 @@
       case V7_7:
         return "blacktop/elasticsearch:7.7.1";
       case V7_8:
-        return "blacktop/elasticsearch:7.8.0";
+        return "blacktop/elasticsearch:7.8.1";
     }
     throw new IllegalStateException("No tests for version: " + version.name());
   }