GitHub Actions are pinned to versions targeting the deprecated Node 20 runtime #14

Describe the bug

`actions/checkout@v4`, `actions/setup-node@v4`, `actions/cache@v4`, and `pnpm/action-setup@v4` all target the Node 20 runtime, which GitHub is deprecating for Actions. CI runs show:

```
##[warning]Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-node@v4, pnpm/action-setup@v4.
```

This is unrelated to the Node 22 used to actually build the site — it's about the runtime the action scripts themselves execute on — but it's a real warning that will eventually become a hard failure once GitHub drops the compatibility shim.

Expected behavior

Actions pinned to their v5 releases, which target the Node 24 runtime.

Additional context

Fixed in #9.