Fix kubelet.conf server line indentation (#13277)

[longxiucai]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Fix invalid YAML indentation in kubelet.conf caused by hardcoded 4-space indent in lineinfile module.
The original task breaks the structure of kubelet.conf when the file uses different indentation levels.
This change uses regex back-reference to preserve original indentation.

Which issue(s) this PR fixes:

Fixes #13277

Special notes for your reviewer:

• Uses backrefs: true to ensure group capture works
• Regex matches any indentation before "server: https"
• Safe for all indentation styles
• Only affects worker nodes when external LB is enabled

Does this PR introduce a user-facing change?:

NONE

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: longxiucai / name: longyuxiang (1fb0b68)

[k8s-ci-robot]

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[tico88612]

Could you fix your CLA?

[tico88612]

/ok-to-test

[yankay]

Thanks @longxiucai
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: longxiucai, yankay

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[Copilot]

Pull request overview

Fixes a kubeadm worker-node upgrade failure where the lineinfile task could rewrite the server: entry in kubelet.conf at an incorrect YAML indentation level, breaking kubeconfig parsing and causing kubelet to crash-loop.

Changes:

• Updates the lineinfile regex to capture and reuse the existing indentation on the server: line.
• Enables backrefs: true so the replacement line preserves the matched indentation instead of hardcoding spaces.

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