Compatibility with next xgboost release

[david-cortes]

The next release of xgboost (likely to come in the following weeks) will have many breaking changes, including removal of the slice method for DMatrices.

In order to ensure a smooth transition, would be ideal if an updated version of this package could be submitted to CRAN, using DMatrix[...] instead of slice, so as to be compatible with both the old and the new versions of xgboost, and removing the import for slice from NAMESPACE.

See https://github.com/dmlc/xgboost/issues/11430 for more info.