[MSVC][std:c++20] flatbuffers failed to detect c++ standard on MSVC

[assorted]

The following code is utilized by FlatBuffers for detecting the language standard:

#ifndef FLATBUFFERS_USE_STD_SPAN
  // Testing __cpp_lib_span requires including either <version> or <span>,
  // both of which were added in C++20.
  // See: https://en.cppreference.com/w/cpp/utility/feature_test
  #if defined(__cplusplus) && __cplusplus >= 202002L
    #define FLATBUFFERS_USE_STD_SPAN 1
  #endif
#endif // FLATBUFFERS_USE_STD_SPAN

This results in a build error when utilizing flatbuffers::span, both with MSBuild and CMake.
Microsoft uses a fake value __cplusplus == 199711L for compatibility, and they advise using _MSVC_LANG instead

[assorted]

MR: #8155

[assorted]

Merged