Build Qt with C++2a

For some reason Qt 6 does not autodetect the latest C++ support by the compiler, instead it requires an opt-in switch to cmake / configure.

This switch is not currently being passed on the CI. The result is that Qt actually FTBFS if C++ 20 gets enabled (some C++ 20 specific codepath was never tested and had errors).

Proposed solution:

• install recent enough compilers on the CI (GCC >= 10, MSVC 2019-latest, clang >= 12, etc.)
• Build as C++20:
  1. either restore the autodetection, and install recent enough compilers on the CI, so we can catch such mistakes, or
  2. manually pass the relevant switch to configure/cmake so to enable C++20 on the toolchains that do support it.