upper cap sklearn to <1.6.0 due to xgboost not supporting it

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog. (not required)

Summary

• scikit-learn released version 1.6.0 and xgboost does not support it yet
• fixes failing unit test checks by upper-capping sklearn to 1.6.0
• we'll relax this once xgboost has added support

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.10%. Comparing base (51b026b) to head (10e8e57).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2618      +/-   ##
==========================================
- Coverage   94.15%   94.10%   -0.06%     
==========================================
  Files         139      139              
  Lines       14992    14992              
==========================================
- Hits        14116    14108       -8     
- Misses        876      884       +8     

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