DEPR: automatic upcasting on invalid reindex fill_value

[jbrockmendel]

ser = pd.Series([1, 2, 3])

>>> ser.reindex([4, 5, 6], fill_value="foo")
4    foo
5    foo
6    foo
dtype: object

PDEP-6 is moving us away from automatic upcasting in __setitem__-like operations. #53868 and #53802 suggest doing the same (with possible int->float exception) for unstack and shift, respectively. I think that just leaves reindex. Maybe we should call this behavior a footgun and deprecate it across the board?