Elasticsearch: Remove support for EOL 6.6 version

As that version is now EOL as of July 29, 2020 ([1]).

Update the config-gerrit documentation accordingly, by linking to the
current Elasticsearch documentation now.

[1] https://www.elastic.co/support/eol

Change-Id: Ied980886440869cc5cdbfc1dd915c0375a2f36e1
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 006af86..7cf3d6a 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -3047,9 +3047,10 @@
 When security is enabled in Elasticsearch, the username and password must be provided.
 Note that the same username and password are used for all servers.
 
-For further information about Elasticsearch security, please refer to the documentation:
-
-* link:https://www.elastic.co/guide/en/elastic-stack-overview/6.6/security-getting-started.html[Elasticsearch 6.6]
+For further information about Elasticsearch security, please refer to
+link:https://www.elastic.co/guide/en/elasticsearch/reference/current/security-getting-started.html[the documentation].
+This is the current documentation link. Select another Elasticsearch version
+from the dropdown menu available on that page if need be.
 
 [[elasticsearch.username]]elasticsearch.username::
 +