CI: update actions to switch to Node.js 20

[hagenw]

As discussed at https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/, we need to update all Github Actions that are still using Node.js 16.

This can also be seen when inspecting a random Actions page:

We fix this here by updating to:

• actions/checkout@v4
• actions/cache@v4
• actions/setup-python@v5
• codecov/codecov-action@v4
• softprops/action-gh-release@v2

[hagenw]

Oh, sorry, I didn't wanted to merge this yet.

If you don't agree with the changes, we can revert them.