remove dask-xgboost, use xgboost 1.3.0+

[jameslamb]

This PR accompanies saturncloud/examples#50. It proposes removing dask-xgboost the next time we build images, and replacing it with xgboost>=1.3.0. The newest release of xgboost contains an XGBoost-maintained Dask integration. I've tested that it works on Saturn Dask clusters, so I think we can safely stop recommending dask-xgboost and move to xgboost.

[rikturr]

The day has arrived!