chore: bump GitHub Actions to fix Node 20 deprecation warnings

Pins actions/checkout v7.0.1, actions/upload-artifact v7.0.2, actions/download-artifact v8.0.2 and
Swatinem/rust-cache v2.9.2 by commit. Re-applied onto current main by the maintainer (the workflows
changed since the PR was opened); dtolnay/rust-toolchain stays on @stable, because pinned to a commit
it needs an explicit toolchain input, and it is a composite action with no Node runtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

.github/workflows/pages.yml

@@ -28,11 +28,11 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: wasm32-unknown-unknown
-      - uses: Swatinem/rust-cache@v2
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
       - name: Install mdbook and trunk
         run: |
           mkdir -p "$HOME/.local/bin"