Fix #94: Fix and clean up CI with supported Ubuntu version and more

[samuell]

This PR fixes #94 by fixing these things:

• Updates the Ubuntu version to a supported (and long term support (LTS) one): 24.04

It also:

• Updates the micromamba action to v2, using the best practice way of installing it.
• Cleans up the flake8 command to only run on the emu library and tests folder.
• Restricts the CI to run only on pull requests, not every push.

[samuell]

(Had to do a rebase+force-push as had somehow not gotten the latest changes before. Should be up to date now!)

[kdcurry]

Thanks @samuell !