kubeadm: Remove kubeadm alpha preflight subcommand

[fabriziopandini]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
After #73163 merged pre-flight checks for kubeadm join are now exposed as phase, so it is possible to remove kubeadm alpha preflight and its own only subcommand kubeadm alpha preflight node

Which issue(s) this PR fixes:
Fixes:kubernetes/kubeadm#1386

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

kubeadm: `kubeadm alpha preflight` and `kubeadm alpha preflight node` are removed; you can now use `kubeadm join phase preflight`

/sig cluster-lifecycle
/priority backlog
@kubernetes/sig-cluster-lifecycle-pr-reviews

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini

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[neolit123]

thanks!
/lgtm

[neolit123]

/hold
holding because i forgot to ask - we might want a release note here, even if we are removing an alpha command, correct?

please feel free to hold cancel @fabriziopandini

[fabriziopandini]

@neolit123 good point. Added a release note
/hold cancel