CI: Address warnings from GitHub Actions

[PMeira]

1. More urgent, but should be trivial to update:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, julia-actions/setup-julia@v1, codecov/codecov-action@v1. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

2. New warning just now:

[julia-buildpkg] Caching of the julia depot was not detected
Consider using `julia-actions/cache` to speed up runs https://github.com/julia-actions/cache. To ignore, set input `ignore-no-cache: true`