Drop Python 3.8 support

[veenstrajelmer]

• python 3.8 is EOL in October 2024: https://devguide.python.org/versions
• many packages already dropped support, overview in: drop Python 3.8 support dfm_tools#267
• update supported python versions in pyproject.toml
• update readme/contributing/installation
• HMC currently uses Python 3.8.14 and RedHat8 does not support python 3.9. Wait for HMC update to RedHat 9 (update 28-5-2024: is being tested, all systems will be transfered somewhere after summer 2024)

Waiting issues:

• timestep_min None instead of np.nan #156
• mindeps yml was made quite complex to make requirement solving on py38 possible by exluding some optional dependencies. In py3.9 this will be easier since toml-to-requirements>=0.2.0 (only available for py>=39) supports including only some of the optional requirement lists