Automatic merge from submit-queue (batch tested with PRs 63783, 63734). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Rename `kubeadmapiext` to the more explicit `kubeadmapiv1alpha1`

**What this PR does / why we need it**:

`kubeadmext` is somewhat confusing to those who read the code (although it means "the external API of kubeadm", which to some degree makes sense), so I'm swapping all references to it to the more explicit `kubeadmapiv1alpha1`. This change is needed given that we will support multiple external APIs.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Part of https://github.com/kubernetes/community/pull/2131

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```
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