Update GitHub Actions to latest versions

[apetro2]

Fixes deprecated GitHub Actions and updates dependencies to prevent future CI failures.

Critical fixes:

• Replace deprecated set-output command with $GITHUB_OUTPUT (will stop working soon per GitHub's deprecation
  notice)
• Replace unmaintained actions-rs/audit-check with actively maintained rustsec/audit-check@v2

Dependency updates:

• actions/checkout: v1/v2/v3 → v4 (standardize across all workflows)
• Swatinem/rust-cache: v1 → v2 (performance improvements)
• styfle/cancel-workflow-action: 0.9.1 → 0.12.1 (bug fixes)

---

Note

Standardizes workflows on latest Actions (checkout v4, rust-cache v2, cancel-workflow-action 0.12.1), replaces deprecated set-output with GITHUB_OUTPUT, and migrates cargo audit to rustsec/audit-check v2.

• CI workflows (.github/workflows/ci.yml):
  • Upgrade actions/checkout to v4 and Swatinem/rust-cache to v2 across jobs.
  • Bump styfle/cancel-workflow-action to 0.12.1.
• GitHub Pages:
  • gh-pages.yml: use actions/checkout@v4; replace deprecated set-output with $GITHUB_OUTPUT.
  • gh-pages-meta.yml: use actions/checkout@v4; bump cancel-workflow-action to 0.12.1.
• Nightly cargo audit (nightly-cargo-audit.yml):
  • Switch from actions-rs/audit-check@v1 to rustsec/audit-check@v2 and use actions/checkout@v4.
• Publish nightly channel (publish-nightly-channel.yml):
  • Use actions/checkout@v4 and $GITHUB_OUTPUT for outputs.
• Update Compiler Explorer (update-compiler-explorer.yml):
  • Use actions/checkout@v4.

^{Written by Cursor Bugbot for commit 8dce939. This will update automatically on new commits. Configure here.}

[fuel-cla-bot]

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