DEPR: remove deprecated date casting; closes #21359
- [x] closes #21359
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Not blocking, but in #21359 the discussed plan was to remove in 0.24.0 Reading through https://github.com/pandas-dev/pandas/issues/21359, it seems like @jreback and @jorisvandenbossche preferred a bit longer of a deprecation. I don't have a preference. this is probably mergable for 1.0, can you rebase Rebased Thanks.