Enforce Kubernetes version skew policy between control plane and workers (MachineSetPreflightChecks)

When upgrading Kubernetes version on a Sylva cluster, no mechanism ensures that the Kubernetes version skew policy is always guaranteed.

In particular the worker nodes (MachineDeployments) must not be upgraded before the whole control plane is upgraded.

Ref: https://kubernetes.io/releases/version-skew-policy/#kubelet

kubelet must not be newer than kube-apiserver.

Some control should be added in Sylva for this purpose when applying a new Kubernetes version.

We can mention also an experimental feature of ClusterAPI that may be helpful: https://cluster-api.sigs.k8s.io/tasks/experimental-features/machineset-preflight-checks