drop Python 3.8 support

[veenstrajelmer]

• xarray dropped py3.8 support in version 2023.02.0 (07-02-2023)
• scipy dropped py3.8 support in version 1.11.0 (25-06-2023)
• numpy dropped py3.8 support in version 1.25.0 (17-06-2023)
• pandas dropped py3.8 support in version 2.1.0 (30-08-2023)
• dask dropped py3.8 support in version 2023.5.1 (26-05-2023)
• matplotlib dropped py3.8 support in version 3.8 (15-09-2023)
• xugrid dropped py3.8 support in version 0.7.1 (17-11-2023)
• copernicus-marine-client also does not support py38
• python 3.8 is EOL in October 2024: https://devguide.python.org/versions/
• py39 suggestion in installation/contributing guide was introduced in June 2023, wait at least a few months before dropping py38 support. Also check py38 percentage at https://pypistats.org/packages/dfm-tools
• py3.11 suggestion in installation/contributing since 30 Sept 2023, just to be safe for the near future. Py 3.11 is faster than py 3.10

Also:

• update supported python versions in pyproject.toml (classifiers and requires-python)
• remove pandas version restriction for py38 from pyproject.toml (and simplify xarray/dask requirement)
• remove pytest-py38.yml workflow and badge in readme
• set minimal xarray version #576