Drop Python 3.8 support

[soxofaan]

Ticket to collect and track struggles/reasons to drop support (or not) for Python 3.8 (officially end of life since 2024-10-07)

related:

• Drop support for Python 3.8 openeo-python-client#717
• Drop Python 3.8 support openeo-geopyspark-driver#1060

[soxofaan]

Being stuck at 3.8 is getting frustrating to be honest.
E.g. for #394 I want to leverage the cube extension support in pystac that was added with version 1.9 in Oct 2023 (!), but I can't because pystac already dropped python 3.8 support back then. So I have to go for tedious error-prone ad-hoc parsing, instead of just using a widely-tested and future-proof library.