ElasticContainer: Upgrade V6_8 to elasticsearch 6.8.13

Change-Id: I8a1afb23a46c4765d51e737150be236644dcf01f
diff --git a/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java b/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
index fb9b2de..48295ea 100644
--- a/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
+++ b/javatests/com/google/gerrit/elasticsearch/ElasticContainer.java
@@ -40,7 +40,7 @@
   private static String getImageName(ElasticVersion version) {
     switch (version) {
       case V6_8:
-        return "blacktop/elasticsearch:6.8.12";
+        return "blacktop/elasticsearch:6.8.13";
       case V7_0:
         return "blacktop/elasticsearch:7.0.1";
       case V7_1: