Merge branch 'john_long-remove-centos6-from-req-docs' into 'master'

Remove centos6 and link to supported os docs

See merge request gitlab-org/gitlab!50659

doc/install/requirements.md

@@ -16,7 +16,7 @@ as the hardware requirements that are needed to install and use GitLab.
 
 - Ubuntu (16.04/18.04/20.04)
 - Debian (9/10)
-- CentOS (6/7/8)
+- CentOS (7/8)
 - openSUSE (Leap 15.1/Enterprise Server 12.2)
 - Red Hat Enterprise Linux (please use the CentOS packages and instructions)
 - Scientific Linux (please use the CentOS packages and instructions)
@@ -35,6 +35,9 @@ For the installation options, see [the main installation page](README.md).
 Installation of GitLab on these operating systems is possible, but not supported.
 Please see the [installation from source guide](installation.md) and the [installation guides](https://about.gitlab.com/install/) for more information.
 
+Please see [OS versions that are no longer supported](https://docs.gitlab.com/omnibus/package-information/deprecated_os.html) for Omnibus installs page
+for a list of supported and unsupported OS versions as well as the last support GitLab version for that OS.
+
 ### Microsoft Windows
 
 GitLab is developed for Linux-based operating systems.