Replace `SubwordTokenizer` with `WordPieceVocabulary`

[sarahyurick]

In cuDF 24.06, SubwordTokenizer will be deprecated in favor of WordPieceVocabulary. We should update https://github.com/rapidsai/crossfit/blob/main/crossfit/op/tokenize.py accordingly.

Relevant PR: rapidsai/cudf#18334