Avoid private mask cudf API in favor of Column.as_mask (#4758)

With cudf releasing and further developing `pylibcudf` this release, I'm starting to look at removing cudf old Cython layer (in `cudf._lib`)

Luckily I only found one usage in cugraph that is replaceable with a public API.

closes #4823

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)
  - Alex Barghi (https://github.com/alexbarghi-nv)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: #4758

Author: mroeschke