Automatic merge from submit-queue (batch tested with PRs 49019, 48919, 49040, 49018, 48874)
kubeadm: Remove the old KubernetesDir envparam
**What this PR does / why we need it**:
The last piece of https://github.com/kubernetes/kubernetes/issues/48053 for moving the env params into the API, and the KubernetesDir into a constant.
After this pretty mechanical change, we might offer short-hand functions in constants like `GetStaticPodManifestDir()` etc easily in order to centralize the `filepath.Join` logic into one place.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
fixes https://github.com/kubernetes/kubeadm/issues/326
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
cc @kubernetes/sig-cluster-lifecycle-pr-reviews @fabriziopandini
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