How to fix CoreOS "WARNING: This system is using cgroups v1"

Problem:

When logging into your CoreOS instance, you see this warning message:

############################################################################
WARNING: This system is using cgroups v1. For increased reliability
it is strongly recommended to migrate this system and your workloads
to use cgroups v2. For instructions on how to adjust kernel arguments
to use cgroups v2, see:
https://docs.fedoraproject.org/en-US/fedora-coreos/kernel-args/

To disable this warning, use:
sudo systemctl disable coreos-check-cgroups.service
############################################################################

but when you look at https://docs.fedoraproject.org/en-US/fedora-coreos/kernel-args/ you only see an example of how to initialize a new CoreOS instance with Ignition files with cgroups v2.

Solution

In order to migrate your system to cgroups v2, run

sudo rpm-ostree kargs --delete=systemd.unified_cgroup_hierarchy

After that, you need to reboot your system in order for the changes to take effect:

sudo systemctl reboot

After the system has rebooted, the error should disappear.