DEPR: enforce deprecation on floats with unit=M or Y
Unfortunately just found that we missed the analogous behavior in to_datetime with non-scalars.  That'll go in the just-do-in-2.0 pile.

Whatsnew is in 1.6 on the assumption it will be changed to 2.0.
@phofl gentle ping, getting this in will allow us to move the whatsnew notes over to the new section and start getting those in thx @jbrockmendel 