contrib: qt: update to 6.8.1 from 6.8.0

Qt 6.8.1 was released today: https://www.qt.io/blog/qt-6.8.1-released. It reportedly contains "more than 550 bug fixes", and some of them affect us.

Qt 6.8.2 is planned to be released in 23.01.2025.

It did not need me to modify the patches.

I have tested on Windows 7 and Windows 10.

Request review @chub.