contrib: qt: update to 6.8.2 from 6.8.0

I already proposed to update to 6.8.1 in !6466 (closed), but it was not realized. Now I'm proposing to update to Qt 6.8.2.

Qt 6.8.2 was released on 31 January 2025 (https://www.qt.io/blog/qt-6.8.2-released). It reportedly contains "more than 450 bug fixes", and some of them affect us. Combined with Qt 6.8.1 update, it would be more than 1000 bug fixes.

Qt 6.8.3 is planned to be released on 18.03.2025 [1], being the last open source LTS patch release. Qt 6.9.0 is planned to be released on 18.03.2025 [2], I think that it will make more sense to update to Qt 6.9 than to stay with Qt 6.8 especially considering that the commercial patches delayed by one year.

I have tested on Windows 7 and Windows 10.

[1] https://wiki.qt.io/Qt_6.8_Release [2] https://wiki.qt.io/Qt_6.9_Release

Request review @chub.