As part of discussions with Stefan during the MIDAS workshop,
an update to supported versions of c++ on different platforms.

- el6/SL6 is gone, gcc 4.4.7, no c++11
- el7/CentOS-7 is out the door, gcc 4.8.5, full c++11, no c++14, no c++17
- el8 was a still born child of RedHat
- el9  - gcc 11.5 with 12, 13, and 14 available, incomplete c++26, c++23, experimental c++20, default c++17
- el10 - gcc 14.2, incomplete c++26, c++23, experimental c++20, default c++17

- U-18 - gcc  7.5, experimental, incomplete c++17
- U-20 - gcc  9.4 default, 10.5 available, experimental, incomplete c++17
- U-22 - gcc 11.4 default, 12.3 available, default c++17
- U-24 - gcc 13.3 default, 14.2 available, default c++17

- D-11 - gcc 10.2, experimental c++17
- D-12 - gcc 12.2, default c++17

- MacOS 15.2 - llvm/clang 16, default c++17
- MacOS 15.7 - llvm/clang 17, default c++17

Taken from C++ level support:

GCC C++ support: https://gcc.gnu.org/projects/cxx-status.html
LLVM clang c++ support: https://clang.llvm.org/cxx_status.html
GLIBC c++ support: https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html

This suggests that c++17 is available on all current platforms.

K.O.

P.S. ROOT moved to C++17 as of release 6.30 on November 6, 2023.