[MNT] extend CI to newer python versions, deprecate python versions 3.8, 3.9 after EOL

[fkiraly]

It seems the CI runs only on python versions 3.8 and 3.9 both of which have already reached end of life.

This PR updates the python versions, deprecating any logic that runs tests on python versions 3.8 and 3.9, or scikit-learn versions of that age.

The test matrix is updated to the currently supported python versions, 3.10-3.14.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.16%. Comparing base (3380bbb) to head (62c0651).
⚠️ Report is 159 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (3380bbb) and HEAD (62c0651). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3380bbb)	HEAD (62c0651)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1571       +/-   ##
===========================================
- Coverage   85.24%   64.16%   -21.08%     
===========================================
  Files          38       36        -2     
  Lines        5008     4317      -691     
===========================================
- Hits         4269     2770     -1499     
- Misses        739     1547      +808     

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[fkiraly]

FYI @PGijsbers

[fkiraly]

superseded by #1579