(fix): bound sklearn because of dask-ml on the release candidate

[ilan-gold]

• See failure in https://github.com/scverse/scanpy/pull/3392/checks?check_run_id=33968110793 and Compatibility with scikit-learn 1.6 dask/dask-ml#1002
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.59%. Comparing base (7131500) to head (ae88f5d).
Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3393      +/-   ##
==========================================
+ Coverage   76.50%   76.59%   +0.09%     
==========================================
  Files         111      111              
  Lines       12874    12874              
==========================================
+ Hits         9849     9861      +12     
+ Misses       3025     3013      -12     

see 1 file with indirect coverage changes

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