Add deprecation warning for CentOS 6

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

changelogs/unreleased/5614-centos-6-deprecation.yml

+---
+title: Add deprecation warning for CentOS 6
+merge_request: 4596
+author:
+type: deprecated

files/gitlab-cookbooks/package/libraries/omnibus_helper.rb

@@ -103,7 +103,8 @@ class OmnibusHelper
     # example: deprecated_os = { 'raspbian-8' => 'GitLab 11.8' }
     {
       'raspbian-9' => 'GitLab 13.4',
-      'debian-8' => 'GitLab 13.4'
+      'debian-8' => 'GitLab 13.4',
+      'centos-6' => 'GitLab 13.7'
     }
   end