mkosi: drop CentOS 8 from CI

It no longer builds due to packaging issues, drop it,
CentOS 9 is still covered

(cherry picked from commit 975961f)

Author: bluca

.github/workflows/mkosi.yml

@@ -68,8 +68,6 @@ jobs:
             release: tumbleweed
           - distro: centos
             release: "9"
-          - distro: centos
-            release: "8"
 
     env:
       SYSTEMD_LOG_LEVEL: debug