[ko] Update kubeadm install guide to use new apt repository and keyring path

[daeun-ops]

Description

This PR updates the Korean kubeadm installation guide to use the new community-maintained apt repository and GPG keyring path, fixing issues related to deprecated apt-key usage.

Issue

Closes: #43628

[k8s-ci-robot]

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[seokho-son]

Hi @daeun-ops, thank you for the contribution! 🙂

PTAL at the following comments.

Also, please make sure the website passes the build test in your local development environment before opening a PR (see the build failure below).

[seokho-son]

Hi @daeun-ops,
For a localization PR, please avoid making changes to content/en/. Let’s keep the focus on content/ko/.

[seokho-son]

This file is also not relevant to the localization PR.

[seokho-son]

/hold

[k8s-ci-robot]

PR needs rebase.

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