DEPR: DatetimeIndex.snap()
we have a ``DatetimeIndex.snap``, this is duplicative of our more general ``.ceil()``, ``.floor()`` and ``.round()`` (and ``.to_period().to_timestamp()``); the former are also generally available on all of the timelike indices (DTI, TDI, PI)

we should remove this and prefer the 3 rounding methods