Automatic merge from submit-queue (batch tested with PRs 52119, 52306)
kubeadm: Mark self-hosting alpha in v1.8
**What this PR does / why we need it**:
Self-hosting is alpha in v1.8, not beta. We targeted it to be beta, hence the initial add of this feature gates' value, but now changing back to alpha.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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