Document removal of kube-dns support in kubeadm

[fabriziopandini]

Kubeadm is going to drop support for kube-dns in version 1.21 (xref kubernetes/kubeadm#1943).

This should not be a problem for CABPK/KCP users, given that almost all of them should be using CoreDNS, however, we should consider if/how to issue a note to our user in case someone is still using kube-dns by twicking KubeadmConfig.Cluster configuration.

/kind feature

[fabriziopandini]

/remove-kind feature
/kind documentation
/area bootstrap
/area control-plane

/milestone v0.4.0

[furkatgofurov7]

@fabriziopandini hi! Has someone started working on this? I am happy to help if no one is interested. (Suggestions/pointers where to start looking and where notes must be put would be awesome to have).

[vincepri]

/good-first-issue
/help
/priority backlog
/milestone v0.4.x

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[sgirem463]

@fabriziopandini Hi, I am a new contributor, would like to work on this. Would appreciate your suggestions/pointers.

[sgirem463]

/assign @sgirem463

[fabriziopandini]

What about adding a note in https://cluster-api.sigs.k8s.io/tasks/kubeadm-control-plane.html#kubeconfig-management. Something like

<aside class="note warning">
<h1>Warning</h1>
KCP support only CoreDNS as a DNS server; also kube-dns support has been removed by kubeadm in v1.21.
</aside>

[sgirem463]

@fabriziopandini I will work on this

[sgirem463]

@fabriziopandini @vincepri
I have made changes to the document source, the diffs are:

ALLANs-MBP:tasks alue$
ALLANs-MBP:tasks alue$ git diff
diff --git a/docs/book/src/tasks/kubeadm-control-plane.md b/docs/book/src/tasks/kubeadm-control-plane.md
index 50b606cf5..d24ab4f3c 100644
--- a/docs/book/src/tasks/kubeadm-control-plane.md
+++ b/docs/book/src/tasks/kubeadm-control-plane.md
@@ -2,6 +2,14 @@

Using the Kubeadm control plane type to manage a control plane provides several ways to upgrade control plane machines.

+

+
+

Warning

+
+KCP support only CoreDNS as a DNS server; also kube-dns support has been removed by kubeadm in v1.21.
+
+
+

Kubeconfig management

KCP will generate and manage the admin Kubeconfig for clusters. The client certificate for the admin user is created
ALLANs-MBP:tasks alue$
ALLANs-MBP:tasks alue$

I am not sure what tests should be done for document updates.
I did "make serve-book" to serve a api book locally, the screenshot of the page is:

Is there anything else I should do before pulling a PR?

[sbueringer]

@sgirem463 I think it's totally fine to just open a PR to propose this change.