Bump C++ standard to C++20

[JanVogelsang]

This PR bumps the C++ standard from C++17 to C++20 which is required to use C++ concepts and modules.

This enables a bunch of follow-up code improvements:

• Stopwatch simplification: Using concepts instead of complex template logic (Modernize the stopwatch implementation #37)
• CMake-to-C++ interface: Using compile-time-constants (constexpr) instead of C-macros (#3399)
• std::format instead of String::compose (Replace String::compose by std::format #38)
• std::variant instead of boost::any (Move from boost::any to std::variant #34)
• KernelManager refactoring and include-tree cleanup enabled by link-time optimization (LTO) (#3544)
• C++20 modules to reduce compile-time

[heplesser]

Hi @JanVogelsang, Given the problem that popped up with format, I would like to discuss this more widely. Please re-open nest#3581 and put it on the agenda for the next NEST Open VC.