DEPR: PeriodIndex.astype(dt64)
- [ ] closes #xxxx
- [x] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Match the Series/EA behavior
web and docs failing

```
2021-11-11T21:46:19.7568073Z WARNING: 
2021-11-11T21:46:19.7569269Z >>>-------------------------------------------------------------------------
2021-11-11T21:46:19.7570278Z Warning in /home/runner/work/pandas/pandas/doc/source/user_guide/timeseries.rst at block ending on line 2098
2021-11-11T21:46:19.7571558Z Specify :okwarning: as an option in the ipython:: block to suppress this message
2021-11-11T21:46:19.7573968Z ----------------------------------------------------------------------------
2021-11-11T21:46:19.7576034Z <ipython-input-382-a92cd46ab454>:1: FutureWarning: Converting PeriodIndex to DatetimeIndex with 'astype' is deprecated and will raise in a future version. Use `obj.to_timestamp(how).tz_localize(dtype.tz)` instead.
2021-11-11T21:46:19.7577328Z   pi.astype("datetime64[ns]")
2021-11-11T21:46:19.7578226Z <<<-------------------------------------------------------------------------
``` Yah, this was premature.  I didn't fix the docbuild bc I was having second thoughts.  Trouble is that we _do_ allow dt64.astype(period), and we should either have both or neither.  The part I have a strong opinion about is that IF we allow period.astype(dt64), it should be in PeriodArray.astype, not in PeriodIndex.astype ok can revert for now if like