now always set to "systemd" unless the user is explicit about it. This requires existing machine setups to configure the container runtime to use the "systemd" driver. Documentation on this topic can be found here: https://kubernetes.io/docs/setup/production-environment/container-runtimes/. When upgrading existing clusters / nodes using "kubeadm upgrade" the old cgroupDriver value is preserved, but in 1.22 this change will also apply to "upgrade". For more information on migrating to the "systemd" driver...