DEPR: datetimelike.astype(int)
- [x] closes #24381
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Sits on top of #38535
can you merge master do we have sufficient `.view` tests on *Index* ? > do we have sufficient .view tests on Index ?

looks decent, yah I ran into this case of datetime->integer casting while exploring our casting rules (https://github.com/pandas-dev/pandas/issues/22384). @jbrockmendel do you remember the rationale for deprecating this casting? It's not really clear to me why we would in the future disallow datetimelike->integer casting.

This PR doesn't seem to give much reasoning, and the referenced issue is about the bitwidth / sign being ignored, but not sure deprecating altogether is necessarily the fix for that (the bitwidth being ignored *also* applies for integer->integer casting, which we didn't deprecate). https://github.com/pandas-dev/pandas/issues/22384#issuecomment-909660696

>> We currently raise a warning about the dt64 -> int64 that this will raise instead in the future and to use view instead. Do you know what the rationale was for this deprecation?

> The idea is that 'astype' in general means something like 'the same information but in a different format' and that doesn't make sense with i8 <-> M8

 Whoops, sorry, I totally forgot I already asked / you answered in the other thread .. :) Will continue there