kubeadm: add note about a bug in the PublicKeysECDSA FG

The PublicKeysECDSA has been poorly tested and supported and
apparently it had a bug where keys in kubeconfig files were
using RSA even if the FG was true.

Add note about that in the FG section of the kubeadm init doc.

Author: neolit123

content/en/docs/reference/setup-tools/kubeadm/kubeadm-init.md

@@ -174,7 +174,9 @@ as a learner and promoted to a voting member only after the etcd data are fully
 `PublicKeysECDSA`
 : Can be used to create a cluster that uses ECDSA certificates instead of the default RSA algorithm.
 Renewal of existing ECDSA certificates is also supported using `kubeadm certs renew`, but you cannot
-switch between the RSA and ECDSA algorithms on the fly or during upgrades.
+switch between the RSA and ECDSA algorithms on the fly or during upgrades. Kubernetes
+{{< skew currentVersion >}} has a bug where keys in generated kubeconfig files are set use RSA
+despite the feature gate being enabled.
 
 `RootlessControlPlane`
 : Setting this flag configures the kubeadm deployed control plane component static Pod containers