CI: drop CentOS Stream 8 jobs
CI: drop CentOS Stream 8 jobs

We kept CS8 jobs while waiting to get CentOS Stream 9 support ready.

But we have situations where OpenStack projects fail due to lack of new
enough (3.8+) Python in CS8.

Time to make CS8 go away.

Change-Id: I32bef531896777bfb463bb401ebba1d488a16bf9

openstack/kolla,openstack/kolla