DEPR: Categorical.to_dense
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Ideally I'd want to keep the method and change the behavior to (an improved variant of) _internal_get_values, but so it goes.
can you add a release note & link to the deprecation removal issue. ~~rebase again pls~~ /azp run <samp>
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</samp> thanks