Verify that Qt compiles with C++2a

The C++20 standard has been released, and compilers have already initial support for it. Moreover, our configure script allows to compile Qt with C++20. We should ensure that those configurations actually work, or at least compile.

Else we might end up with bugreports like QTBUG-90898.

Ideally we would compile with gcc, clang and MSVC in C++20 mode, but even using only one of those compilers in C++20 mode would already help tremendously in avoiding breakage. Running tests could be useful to catch some divergence, but is less likely to catch anything interesting.