Update GitHub Actions runtime

[CyrusAuyeung]

Summary

Update the two official GitHub Actions used by validation from v4 to v6.

Why

The successful post-merge validation run reported that actions/checkout@v4 and actions/setup-node@v4 target the deprecated Node.js 20 action runtime and were being forced onto Node.js 24 by the hosted runner. Both official actions now publish stable v6 releases for the Node.js 24 runtime.

Impact

• Removes the known action-runtime deprecation warning.
• Keeps the workflow's Node.js 24 project test matrix and npm cache configuration unchanged.
• Does not deploy or modify the VPS.

Validation

• Actionlint 1.7.12
• git diff --check
• GitHub Actions will run the complete repository validation and Docker build with both v6 actions.

Checklist

• No production secret, address, certificate, dump, or runtime data is included
• Validation workflow behavior remains unchanged apart from the action runtime upgrade
• No production deployment job is introduced

[CyrusAuyeung]

Superseded by Dependabot PRs #3 and #4, which update the same official actions directly to the current stable v7 releases and have already passed the complete validation workflow.