CI yaml file should be updated

[chqrlie]

All CI jobs complain about needing an update from node 16 to node 20:

linux (Debug)
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/.

Changing actions/checkout@v3 to actions/checkout@v4 seems an easy fix, but I am not sure if it will be sufficient.

[saghul]

That should indeed be the only change necessary.