Enable C++20

[h4tr3d]

$ avr-gcc --version
avr-gcc (GCC) 14.1.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

GCC 14 has well support for the core C++20 features. Yep, most useful features still in the STL that does not present for AVR but some useful functionality can be ported per-project.

At least, I want to implement FSM with std::variant+std::visit and overload pattern. With O3 optimization level I see no significant penalty in the code and it is interesting to implement it on this platform.