systemd enabling cgroup v2 by default (default-hierarchy=unified)

[Lukas Märdian]

Hi all,

After delaying this for a long time, we want to follow upstream, Debian 
and others in switching to the "unified" cgroup hierarchy (cgroupv2) by 
default:
https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1850667

As discussed with the snapd and release teams, the next systemd upload 
will land this change (before FeatureFreeze). We expect some minor 
outfall in the proposed migration (wrt. snapd), that should be mitigated 
via https://github.com/snapcore/snapd/pull/10589 and resolved later this 
cycle once the full snapd cgroup v2 support is landed:
https://github.com/snapcore/snapd/pulls?q=is%3Apr+is%3Aopen+label%3Acgroupv2

If for some reason you need to keep the legacy cgroup v1 hierarchy, you 
can select it via a kernel parameter at boot time:
systemd.unified_cgroup_hierarchy=0

Cheers,
   Lukas