tox.ini: Add centos-stream-8, centos-stream-9, remove defunct centos-6 #33196
Description
docker pull quay.io/centos/centos:stream8
docker pull quay.io/centos/centos:stream9
etc., see https://quay.io/repository/centos/centos?tab=tags
Component: porting
Author: Matthias Koeppe
Branch/Commit: 4978a3b
Reviewer: Frédéric Chapoton
Issue created by migration from https://trac.sagemath.org/ticket/33196