CI: drop CentOS Stream 8

It has gone EOL a few days ago and the job is failing. CentOS 7 will go
down soon, but we'll keep the CI while it's still working.

It anyone wants to add Rocky Linux into the mix, or any other, PRs are
open :-D

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

.github/workflows/tests.yml

@@ -27,7 +27,6 @@ jobs:
           - {name: "archlinux", tag: "latest", variant: "-lts"}
           - {name: "archlinux", tag: "latest", variant: "-zen"}
           - {name: "centos/centos", tag: "stream9", url: "quay.io/"}
-          - {name: "centos/centos", tag: "stream8", url: "quay.io/"}
           - {name: "centos", tag: "7"}
           - {name: "debian", tag: "testing"}
           - {name: "debian", tag: "12"}