Note in docs/version check < ES 6.4 is not supported

In the following MRs:

1. https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35292
1. https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33176

We broke support support for these previous version by introducing
features that are only available in 6.4+ and 6.2+
https://gitlab.com/gitlab-org/gitlab/-/issues/271218 .

The simplest option here seems to just be to document that it's
unsupported rather than trying to fix these since we're going to remove
support soon anyway for all of `6.x`.

doc/integration/elasticsearch.md

@@ -23,8 +23,9 @@ and the advantage of the following special searches:
 
 | GitLab version                              | Elasticsearch version         |
 |---------------------------------------------|-------------------------------|
-| 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 13.6 or greater   | Elasticsearch 7.x (6.4 - 6.x deprecated to be removed in 13.8) |
+| GitLab Enterprise Edition 13.2 through 13.5 | Elasticsearch 6.4 through 7.x |
+| GitLab Enterprise Edition 12.7 through 13.2 | 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 7.x (6.x deprecated to be removed in 13.8)?'
+      set_name 'Elasticsearch version 7.x (6.4 - 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})" }
@@ -22,7 +22,7 @@ module SystemCheck
       def check?
         case self.class.current_version.major
         when 6
-          true
+          self.class.current_version.minor >= 4
         when 7
           true
         else

ee/spec/lib/system_check/app/elasticsearch_check_spec.rb

@@ -34,7 +34,8 @@ RSpec.describe SystemCheck::App::ElasticsearchCheck do
       '5.3.1' | false
       '5.6.0' | false
       '5.6.6' | false
-      '6.0.0' | true
+      '6.0.0' | false
+      '6.3.0' | false
       '6.4.2' | true
       '7.1.0' | true
       '7.5.1' | true