API/REGR: Convert to float for index union
This restores the 0.24.x behavior of Index.union(other) between
Float and (U)Int indexes. These are again floating dtype.

left  | right | output of left.union(right)
----- | ----- | ------
int   |float  | float64
int   |uint   | object
float | uint  | float64

https://github.com/pandas-dev/pandas/issues/26778#issuecomment-505108465

Closes https://github.com/pandas-dev/pandas/issues/26778
