See https://www.redhat.com/sysadmin/fedora-31-control-group-v2 for
details on why this is desirable, and how it impacts containers.
Users that need to keep using the old cgroup hierarchy can re-enable it
by setting `systemd.unifiedCgroupHierarchy` to `false`.
Well-known candidates not supporting that hierarchy, like docker and
hidepid=… will disable it automatically.
Fixes#73800