[Ci-users] reminder : CentOS Linux 8 EOL in december, including for ci test

Just a quick reminder that CentOS Linux 8 is going EOL in december (see https://www.centos.org/centos-linux-eol/)

So based on that plan, we'll also remove it from available ci infra at the same time as on mirror network, and if you'll ask for CentOS 8 you'll get no returned node to run your tests on.

Start switching your tests to run on 8-stream instead, available since the beginning.