Update GitHub Actions to latest stable versions

[Copilot]

Updates workflow dependencies to current stable releases and pins floating version references.

Changes:

• actions/checkout: v3 → v4
• actions/cache: v2 → v4
• actions/stale: @main → v9

The stale workflow now uses a pinned version instead of tracking @main, preventing unexpected breaking changes.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/jeffreytse/jekyll-deploy-action/releases/latest
  • Triggering command: curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

update the build workflows

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.