For C++20 use keyword requires instead of SFINAE magic

[g-217]

My earlier attempt to fix build failures on MSVS 17.6 and C++20 caused build failures on Ubuntu + clang-11.
I have created a PR to revert those changes, #122

Instead of changing existing working code, I am proposing to use C++20 keyword requires to check if << operator exists. With C++20 we can write a simpler check as following:

    if constexpr (requires { s << x; }) {
        s << x;
    }

[sean-parent]

This looks good, but could you also add C++20 test builds to .github/workflows/adobe_source_libraries.yml?