⚠️ Default Kubelet cgroupDriver to systemd for Kubernetes >= 1.21

[fabriziopandini]

What this PR does / why we need it:
For Kubernetes >= 1.21, image builder is going to setup containerd for using systemd cgroup.
KCP should take care of adapting the Kubelet-Config map to be consistent with this change.

Which issue(s) this PR fixes:
Fixes #4099

/area control-plane
/milestone v0.4.0

/hold
for the change in image builder to be merged, rif kubernetes-sigs/image-builder#471

[fabriziopandini]

/retest

[fabriziopandini]

/test pull-cluster-api-test-main

[fabriziopandini]

/hold cancel
PR merged in kubeadm and image builder (thanks @neolit123 & @codenrhoden), so we are ready to go in CAPI as well.
This should be back ported in the release 0.3 branch too
/assign @vincepri @CecileRobertMichon @detiber

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

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