Remove the numpy and pandas version restriction in pyproject.toml

[jeongyoonlee]

Proposed changes

This PR fixes #680 by removing the numpy and pandas version restriction in pyproject.toml. I tested causalml with the latest numpy and pandas locally and it passed the build and all tests.

I also removed ntree_limit from GradientBoostedPropensityModel.predict() to reflect the latest XGBoost API updates and resolve the build error (e.g., https://github.com/uber/causalml/actions/runs/6285485709/job/17079160049?pr=683).

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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)
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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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