Drop Python 3.6 and 3.7 support in CI

[EwoutH]

This commit drops Python 3.6 and 3.7 in the CI configurations. Both versions are end-of-life and aren't supported or updated anymore.

This simplifies the CI configuration and will allow for easier CI upgrades in the future.

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This is part of a multi-PR effort that aims to support modern Python versions and platforms.

1. Removing old Python versions (this PR)
2. Updating the code base to modern Python 3 standards.
3. Updating all the environments and tools used (see #2414)
4. Adding new Python versions (3.11 and 3.12)
5. Adding new platforms (arm64)

[franz101]

Interesting that the tests are failing, one part is the catboost_prediction_function I compiled a list of stable release when vaex 4.17 was released:

python-utils==3.8.1
rich==13.7.0
pyyaml==6.0.1
matplotlib-base==not-found
uvicorn==0.23.2
geopandas==0.13.2
gcsfs==2022.11.0
pytest-mpl==0.16.1
fsspec==2023.1.0
pytz==2023.3
tornado==6.4
pytest==7.4.3
numpy==1.24.4
filelock==3.13.1
progressbar2==4.3.2
pcre==0.1.0
scikit-learn==1.3.2
catboost==1.2
tabulate==0.9.0
joblib==1.3.2
ipyvolume==0.6.3
graphviz==0.20.1
pyarrow==8.0.0
py-xgboost==not-found
aplus==0.11.0
notebook==7.0.6
diskcache==5.6.3
s3fs==2023.1.0
pip==24.0
python-graphviz==999.0.2
cython==0.29.36
pyqt==0.1
astropy==5.2.2
pytest-asyncio==0.21.1
pandas==1.5.3
scipy==1.10.1
zipp==3.17.0
blake3==0.4.1
pytest-timeout==2.2.0
python-annoy==not-found
h5py==3.10.0
plotly==5.18.0
xarray==2023.1.0
cachetools==5.3.2
numba==0.58.1
lightgbm==3.3.5
httpx==0.26.0
nest-asyncio==1.6.0

[EwoutH]

I'm going to close this in favour of ongoing efforts (like #2331).