CentOS Stream build infra is still hosted on rhel8, but it should be at least migrated to RHEL9 (what's now used in Brew too)

The only reason why some builders were still on el8 was due to imagefactory, but normally should be decommissioned soon.

Upgrading to RHEL9 would help us with the running kernel for koji host and the nspawn issue we had (and for which a workaround to disable new chroot style was added)