fix: link checker workflow fails due to missing skip_file.txt and Node.js 20 deprecation

[rubambiza]

Summary

The link_checker.yaml workflow is failing on every run:

• Primary failure: skip_file.txt is referenced via --skip-file but doesn't exist in the repo
• Upcoming breakage: actions/checkout@v4 uses Node.js 20, which will be forced to Node.js 24 on June 16, 2026 and removed September 16, 2026

Failing run (raised by @webchang): https://github.com/kagenti/.github/actions/runs/27018592949/job/79740104037

Fix

1. Add an empty skip_file.txt to the repo root
2. Pin actions/checkout to v6.0.3 SHA (df4cb1c069e1874edd31b4311f1884172cec0e10) for Node.js 24 support