DEPR: Remove NumericIndex.__new__ + related methods
Removes methods `__new__`, `_ensure_array`, `_validate_dtype`, `_ensure_dtype` & `_assert_safe_casting` from `NumericIndex`. Also adds needed contents to `Index` and other locations as needed.

Some scaffolding is added to `NumericIndex.__new__`, but that will be removed in the next PR.

xref #42717.

Thanks @topper-123 