cmake: support setting CMAKE_CXX_STANDARD on the commandline #10609
Description
The way CMAKE_CXX_STANDARD is currently used in CMakeLists.txt, a user cannot override it on the command line invocation of cmake. It would be nice to update things so that users can easily test -std=c++20 builds if they like.
Currently CI has a couple builds that run with -std=c++20 just to ensure we don't break things for C++20. We don't currently support using C++20 features, but we want to make sure we don't use deprecated features that are removed in 20 and thus break the 20 build.