BUG: Float64Index.astype('u8') returns Int64Index
- [x] closes #45151
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

We _could_ deprecate the casting instead of raising.  Or could decide we're OK allowing it I guess.
this is fine