* Deprecate LabelEncoder in XGBClassifier; skip LabelEncoder for cuDF/cuPy inputs
* Add unit tests for cuDF and cuPy inputs with XGBClassifier
* Fix lint
* Clarify warning
* Move use_label_encoder option to XGBClassifier constructor
* Add a test for cudf.Series
* Add use_label_encoder to XGBRFClassifier doc
* Address reviewer feedback