Pin scikit-learn<1.6 until xgboost fix gets released

[jeongyoonlee]

Proposed changes

This PR fixes #808 by pinning scikit-learn to be <1.16 temporarily until xgboost fix gets released.

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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Further comments

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