add C++23 build

Comes from mmutz's comment in https://codereview.qt-project.org/c/qt/qt5/+/441540/comments/84ebd65d_3365c521?tab=comments

I have long said that the most efficient use of CI time would be to build, for each platform, with the oldest supported compiler (gcc 9, clang?) in the oldest supported C++ edition (C+17) and with the resp. newest version of the compiler (GCC 15, Clang 19) with the youngest C+ edition (C+23, but probably C+2c at this point). But I understand that the goal of the CI is not to run the tests, mainly, but to produce packages. And that has completely different constraints.

So if you're asking whether GCC 15 and Clang 19 are good candidates for a C++23 build: yes, they are. But just because they're the resp. latest versions. Once Clang 20 comes out ... actually, it is already out. I just didn't see the branch when building my Clang. And the online docs already have version 21. So Clang 20 at this point, actually. And once Clang 21 comes out, that.

MSVC latest with /std:latest if we don't have that already.

And either significant, or any new warning or build error immediately mirrored to Jira. We had non-Werror C+20 builds for too long, and I kept wondering why I had to fix C+17isms in my local builds.

We should start to test C++23 build.