Briefcase should be marked with at least "partial" support for Qt 6

In https://doc.qt.io/qtforpython/deployment.html and https://doc.qt.io/qtforpython/deployment-briefcase.html#status-of-qt-6-support — briefcase the documentation says that Briefcase does not support Qt 6.

In fact, Briefcase does not support starting templates from scratch for Qt 6 (with the command briefcase new), which is a relatively minor feature.

Everything else works, by indicating "no GUI toolkit" in briefcase new and importing Qt 6 manually in the code. This is mentioned in Briefcase repository: https://github.com/beeware/briefcase/issues/560

Qt documentation should indicate at least "partial" support.