https://pagure.io/centos-infra/issue/1640

While cbs prod builders were migrated to el9 already, we still have our limited .stg. env on el8.
We should just migrate to el9 so that we can try new features/upgrades on that small env before being able to deploy on cbs.centos.org (and so matching same platform)

That will permit also upgrading to 1.35 (see #1639)