contrib: qt: update to 6.8.3 from 6.8.0

I already proposed to update to 6.8.1 in !6466 (closed) and 6.8.2 in !6790 (closed), but they were not realized. Now I'm proposing to update to Qt 6.8.3, with !6996 (merged) along the way we should not need to update the docker image.

Qt 6.8.1 was released on 02 December 2024 (https://www.qt.io/blog/qt-6.8.1-released). It reportedly contained "more than 550 bug fixes".

Qt 6.8.2 was released on 31 January 2025 (https://www.qt.io/blog/qt-6.8.2-released). It reportedly contained "more than 450 bug fixes".

Qt 6.8.3 was released today. Considering the other patch versions, this one also probably has around 500 bug fixes.

Combined with 6.8.1 and 6.8.2, it would be more than 1500 bug fixes. Some of these bugs already affect us.

Qt 6.8.3 is the last LTS patch release without yearly delay. After this, we should decide whether to stay on Qt 6.8 until Qt 6.11 LTS is released, or switch to Qt 6.9 (which should be released until end of this month).

In my opinion, we should switch to Qt 6.9, and continue with Qt 6.10, but this needs a proper discussion.

I have tested on Windows 7 and Windows 10.

Request review @chub.