ci.yml: use actions/checkout@v4 and actions/upload-artifact@v4
This addresses the following warnings from GitHub Actions:
Node.js 16 actions are deprecated. Please update the following
actions to use Node.js 20: actions/checkout@v3. For more information
see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
Node.js 16 actions are deprecated. Please update the following
actions to use Node.js 20: actions/upload-artifact@v3. For more
information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
A near-identical warning forced everyone to upgrade to v3 last year, so
this is some pointless churn, but let's just get it over with again...