Update actions/checkout from v4 to v6

[ElizabethSamuel-MSFT]

Updates the actions/checkout action in the auto-publish workflow from v4 to v6 for consistency with other repos that already use v6.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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[learn-build-service-prod]

Learn Build status updates of commit b56e359:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/workflows/auto-publish.yml	✅Succeeded

For more details, please refer to the build report.

[ElizabethSamuel-MSFT]

@lindalu-MSFT Can you review and merge? This updates the workflow so the dependency action is a version that uses Node.js LTS. Thanks.