kubeadm: promote EtcdLearnerMode to beta

[pacoxu]

What type of PR is this?

/kind cleanup
/kind feature

What this PR does / why we need it:

The CI https://testgrid.k8s.io/sig-cluster-lifecycle-kubeadm#kubeadm-kinder-learner-mode-latest. This feature is alpha since 1.27. I wondered if we could promote it to beta in the v1.29 release cycle.

• docs update: I will do it ASAP.
• feedbacks: RFE: use learner mode for joining etcd members kubeadm#1793 (comment)

Which issue(s) this PR fixes:

xref kubernetes/kubeadm#1793

Special notes for your reviewer:

Does this PR introduce a user-facing change?

kubeadm: promoted feature gate `EtcdLearnerMode` to beta. Learner mode for joining etcd members is now enabled by default.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [KEP]: https://github.com/kubernetes/enhancements/tree/master/keps/sig-cluster-lifecycle/kubeadm/3614-etcd-learner-mode

[k8s-ci-robot]

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[SataQiu]

/lgtm
/hold
for more review
feel free to cancel hold

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 314cf1f539b04b4ec72f029efc2c238a19c9ca9f

[SataQiu]

/cc @neolit123

[neolit123]

/lgtm
/approve

the ci has been stable.
+1 for beta promotion.

[neolit123]

/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: neolit123, pacoxu

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• cmd/kubeadm/OWNERS [neolit123,pacoxu]

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[neolit123]

kubeadm: promote feature gate EtcdLearnerMode to beta

in the RN we can add:

. Learner mode for joining etcd members is now enabled by default.

[sftim]

Changelog suggestion

kubeadm: promoted feature gate `EtcdLearnerMode` to beta. Learner mode for joining etcd members is now enabled by default.

[pacoxu]

Changelog suggestion

kubeadm: promoted feature gate `EtcdLearnerMode` to beta. Learner mode for joining etcd members is now enabled by default.

Updated. Thanks.