Minimum cpp version should be 20 now that span is being used

[anedergaard]

See cpp reference for when span was introduced https://en.cppreference.com/w/cpp/container/span

[ionutneicu]

It would work on C++14 at least.
For this, it requires a std::span version for old C++ compilers such as: https://raw.githubusercontent.com/tcbrindle/span/master/include/tcb/span.hpp plus some other minor changes.
However it won't compile on C++11 due to further constexpr errors which are harder to solve.