The KCM is moving to means of only singing apiserver (kubelet) client
and kubelet serving certificates. See:
https://github.com/kubernetes/enhancements/blob/master/keps/sig-auth/20190607-certificates-api.md#signers
Up until now the experimental kubeadm functionality '--use-api'
under "kubeadm alpha certs renew" was using the KCM to sign *any*
certficate as long as the KCM has the root CA cert/key.
Post discussions with the kubeadm maintainers, it was decided that
this functionality should be removed from kubeadm due to the
requirement to have external signers for renewing the common
control-plane certificates that kubeadm manages.
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