XGBoost example notebook uses deprecated dask-xgboost

This [Dask Examples notebook](https://github.com/dask/dask-examples/blob/main/machine-learning/xgboost.ipynb) uses the deprecated `dask-xgboost` rather than the native XGBoost integration.

Tweaking the notebook should be relatively straightforward by following the example code in [this blog](https://coiled.io/blog/dask-xgboost-python-example/)

```
import xgboost as xgb

# Create the XGBoost DMatrices
dtrain = xgb.dask.DaskDMatrix(client, X_train, y_train)
dtest = xgb.dask.DaskDMatrix(client, X_test, y_test)

# train the model
output = xgb.dask.train(
client, params, dtrain, num_boost_round=4,
evals=[(dtrain, 'train')]
)

# make predictions
y_pred = xgb.dask.predict(client, output, dtest)
```

From the [`dask-xgboost`](https://github.com/dask/dask-xgboost) repo:

"Warning: Dask-XGBoost has been deprecated and is no longer maintained. The functionality of this project has been included directly in XGBoost. To use Dask and XGBoost together, please use xgboost.dask instead https://xgboost.readthedocs.io/en/latest/tutorials/dask.html."