[DOCU-2329] Linux OS support and deprecation (#4000)

* create one topic for all supported and deprecated linux OSes

* remove centos from 3.0 doc and set up redirect

* add links to support tables; move deprecation notices to changelog; remove link blocks

* remove unused 'os_version'

* add note to install doc; fix typo

* group linux distibutions into their own section

* update smoke test

src/gateway/install-and-run/rhel.md

@@ -9,7 +9,7 @@ Kong is licensed under an
 
 ## Prerequisites
 
-* A supported system with root or [root-equivalent](/gateway/{{page.kong_version}}/plan-and-deploy/kong-user) access.
+* A [supported system](/gateway/{{page.kong_version}}/install-and-run/os-support) with root or [root-equivalent](/gateway/{{page.kong_version}}/plan-and-deploy/kong-user) access.
 * (Enterprise only) A `license.json` file from Kong
 
 ## Download and Install

src/gateway/install-and-run/ubuntu.md

@@ -9,7 +9,7 @@ Kong is licensed under an
 
 ## Prerequisites
 
-* A supported system with root or [root-equivalent](/gateway/{{page.kong_version}}/plan-and-deploy/kong-user) access.
+* A [supported system](/gateway/{{page.kong_version}}/install-and-run/os-support) with root or [root-equivalent](/gateway/{{page.kong_version}}/plan-and-deploy/kong-user) access.
 * (Enterprise only) A `license.json` file from Kong
 
 ## Download and install