C++14 requirement mismatch between README and source code

[oliverlee]

README says: C++14 backport (e.g., fold expressions not required)

but I noticed use of fold expressions here: https://github.com/kokkos/mdspan/blob/fef0c8a40e4bd6111eeee36416593504f50cadf3/include/experimental/__p0009_bits/extents.hpp#L61-L63

there may be more (I haven't checked).

Should the use of fold expressions be removed? Or will C++14 support require some C++17 language features?