Move to C++20 (for rpm 6.1)

[pmatilai]

C++20 is one of the more attractive C++ standard versions for our purpose, for example std::string_view and std::span are something that will be hugely useful in modernizing the rpm codebase.

Looking at https://gcc.gnu.org/projects/cxx-status.html, gcc's support for C++20 was pretty complete by gcc-12 already except for modules. So in retrospective we might've just as well started with that because modules aren't interesting to us yet, but it's a bit late to change the baseline for 6.0 now. Lets plan to make this jump post 6.0, probably after 6.0 beta is out to avoid creating too many backport pains during stabilization.