Explicit GCC 13 Support, main branch (2023.09.04.)

Taught AtlasCMake and AtlasLCG to use C++20 by default with GCC 13. Following the convention agreed to with SFT.

With that in place, improved the C++ standard setup of Boost, Eigen and Geant4. Instead of hardcoding the configurations for just a couple of C++ standards, they now use CMAKE_CXX_STANDARD in a much more generic way.

@elmsheus, this should come before !1035 (merged). But the G4 update will even be useful for our current nightlies!