ci: upgrade github action dependencies for required node and pnpm versions

[tukib]

Problem

Our build scripts depend on pnpm/action-setup and actions/setup-node, which rely on Node 16 according to build artifact logs:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: pnpm/action-setup@v2, actions/setup-node@v3, actions/upload-artifact@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

As Node 16 is now EOL in GitHub Actions (since Nov 12), we should pin the dependencies to the Node 20 as recommended (and correctly reflecting our package.json)

Context

pnpm/action-setup@v2 -> pnpm/action-setup@v4
actions/setup-node@v3 -> actions/setup-node@v4

I have also bumped the pnpm version to 9, as this is the correct version for the current checked in lock file (resolves #484).

I've initially submitted these changes together in anticipation that these dependencies are tightly coupled / to prevent spurious errors, and because our local development environments have been using pnpm 9 and Node 20 already without issues.

[tukib]

From what I can tell from the linked repos, no other workflow configuration changes are needed to migrate.