Port Qt backend to Qt 6

[jgcodes2020]

The Qt backend should be ported to Qt 6 ASAP, as the current backend runs on Qt 5, set to be discontinued May 2025.

[ogoffart]

The Qt backend is already ported to Qt6.
It even defaults to Qt6 if both Qt6 and Qt5 are installed.

Although I realized that the linux package are still built with Qt5.
That was because on Linux, most distribution still used KDE that was using Qt5. But I guess now, we should change the package to be built with Qt6.

[jgcodes2020]

I did a bit more digging in the code; seems to be related to woboq/qmetaobject-rs#314. I've submitted a PR that should ensure qttypes defaults to Qt 6 where possible.

[ogoffart]

Indeed, i was thinking we used Qt6 by default but that's only in CMake with C++.
With Rust, qttypes indeed defaulted to whatever 'qmake' is which is most likely Qt5. New release of qttypes was released