Merge branch 'stable-3.1' into stable-3.2

* stable-3.1:
  Fix documentation of `container.slave`
  Update git submodules
  Add support for Elasticsearch version 7.8.*

Change-Id: I2492d62589c7dc959dd5d0c900e3b4abdf92d3ef

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

@@ -28,7 +28,8 @@ public enum ElasticVersion {
   V7_4("7.4.*"),
   V7_5("7.5.*"),
   V7_6("7.6.*"),
-  V7_7("7.7.*");
+  V7_7("7.7.*"),
+  V7_8("7.8.*");
 
   private final String version;
   private final Pattern pattern;