As a developer of Qt, I'd like the CI to run at least some platform integrations in C++20 mode

As a qt developer, whenever I try to compile the project in C++20 mode, I first need to go and fix some compile errors, and there are lots of warnings, too. Of course, I'm done with that at some point, but when I then start adding C++20-only API to Qt, I would hope it gets non-zero test coverage on the CI, too.