systemd-nspawn uses cgroupv1

[Josh Santos]

Even though Ubuntu 22.04 is definitely using cgroup2:
cgroup2 on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)

systemd appears to be using cgroup:

cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/misc type cgroup (rw,nosuid,nodev,noexec,relatime,misc)

When starting a container using default configuration from `systemd-nspawn@` unit it automatically selects cgroupv1 for some reason:

 <email address hidden> - Container debian-sid
     Loaded: loaded (/lib/systemd/system/systemd-nspawn@.service; disabled; vendor preset: enabled)
     Active: active (running) since Sat 2024-07-20 00:48:30 +07; 1s ago
       Docs: man:systemd-nspawn(1)
   Main PID: 5210 (systemd-nspawn)
     Status: "Container running."
      Tasks: 2 (limit: 16384)
     Memory: 2.7M
     CGroup: /<email address hidden>
             ├─payload
             │ └─5214 /usr/lib/systemd/systemd
             └─supervisor
               └─5210 systemd-nspawn --quiet --keep-unit --boot --link-journal=try-guest --network-veth -U --settings=override --ma>

ก.ค. 20 00:48:30 ubuntu-dev systemd[1]: Starting Container debian-sid...
ก.ค. 20 00:48:30 ubuntu-dev systemd[1]: Started Container debian-sid.
ก.ค. 20 00:48:30 ubuntu-dev systemd-nspawn[5210]: Legacy cgroup v1 support selected. This is no longer supported ...

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: systemd-container 249.11-0ubuntu3.12
ProcVersionSignature: Ubuntu 6.5.0-44.44~22.04.1-generic 6.5.13
Uname: Linux 6.5.0-44-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Jul 20 00:43:45 2024
InstallationDate: Installed on 2024-07-19 (0 days ago)
InstallationMedia: Ubuntu 22.04.4 LTS "Jammy Jellyfish" - Release amd64 (20240220)
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)