[FEA] Modify Python digitize functions to use upper_bound and lower_bound

[trevorsm7]

Is your feature request related to a problem? Please describe.
gdf_digitize is being removed from the C++ API as it duplicates the functionality of cudf::upper_bound and cudf::lower_bound defined in #3229. There are Python bindings/wrapper around this function that need to be updated.

Describe the solution you'd like
The Python wrapper can call cudf::upper_bound and cudf::lower_bound instead.

[harrism]

@kkraus14 should this be covered by the Cython libcudf++ port?

[kkraus14]

No, likely will be done in a follow up.

[rgsl888prabhu]

@kkraus14 @harrism #4191 has already incorporated lower_bound and upper_bound in digitize, I think this can be closed.