Office is seeing build breaks due to `#include "span"` including C++20 span instead of gsl/span. Most likely we want all headers includes qualified with "gsl/" to avoid similar issues.
<gsl/string_span>
<gsl/zstring>
With `string_span` having been deprecated (https://github.com/microsoft/GSL/pull/931, https://github.com/microsoft/GSL/pull/945) and removed (https://github.com/microsoft/GSL/pull/1074), the header `<gsl/string_span>` now only contains the definitions for the `zstring` family. Update the name accordingly from `<gsl/string_span>` to `<gsl/zstring>`. The old header is now deprecated and should no longer be used and will be removed in some future release.
