Document support for ES 6.x is deprecated

changelogs/unreleased/deprecate-support-for-es6.yml

+---
+title: Deprecate support for Elasticsearch 6.x
+merge_request: 45619
+author:
+type: deprecated

doc/integration/elasticsearch.md

@@ -23,7 +23,8 @@ and the advantage of the following special searches:
 
 | GitLab version                              | Elasticsearch version         |
 |---------------------------------------------|-------------------------------|
-| GitLab Enterprise Edition 12.7 or greater   | Elasticsearch 6.x through 7.x |
+| GitLab Enterprise Edition 13.6 or greater   | Elasticsearch 7.x (6.x deprecated to be removed in 13.8) |
+| GitLab Enterprise Edition 12.7 through 13.5 | Elasticsearch 6.x through 7.x |
 | GitLab Enterprise Edition 11.5 through 12.6 | Elasticsearch 5.6 through 6.x |
 | GitLab Enterprise Edition 9.0 through 11.4  | Elasticsearch 5.1 through 5.5 |
 | GitLab Enterprise Edition 8.4 through 8.17  | Elasticsearch 2.4 with [Delete By Query Plugin](https://www.elastic.co/guide/en/elasticsearch/plugins/2.4/plugins-delete-by-query.html) installed |

ee/lib/system_check/app/elasticsearch_check.rb

@@ -3,7 +3,7 @@
 module SystemCheck
   module App
     class ElasticsearchCheck < SystemCheck::BaseCheck
-      set_name 'Elasticsearch version 6.x - 7.x?'
+      set_name 'Elasticsearch version 7.x (6.x deprecated to be removed in 13.8)?'
       set_skip_reason 'skipped (elasticsearch is disabled)'
       set_check_pass -> { "yes (#{self.current_version})" }
       set_check_fail -> { "no (#{self.current_version})" }