Use enable_categorical flag in XGBClassifier to avoid encoding categorical features

- With XGBoost 1.5.0, you can now use `enable_categorical` argument to pass categorical data (which avoids us needing to one-hot encode categorical columns)
- https://xgboost.readthedocs.io/en/stable/python/examples/categorical.html#sphx-glr-python-examples-categorical-py

```python
import xgboost as xgb

clf = xgb.XGBClassifier(
enable_categorical=True, max_cat_to_onehot=1
)
```