update Windows Qt to 6.8.3, fixes menus opening on the wrong screen

[hendorog]

Qt 6.2.4 contains a screen-detection regression (QTBUG-101203, fixed in Qt 6.2.5): monitors reporting identical device names (two external monitors of the same model) are treated as a single physical screen. QMenu popups on the affected monitor then position against the wrong screen geometry, appearing detached from the menu bar or on another display entirely. The bug only reproduces with identical-model monitor pairs, which is why reports of it have been inconsistent.

Qt 6.8.3 (LTS) also includes later popup placement fixes (QTBUG-118695, QTBUG-120011) and reworked Windows DPI handling.

[jankae]

I have tried this before and would very much like to build against a newer Qt version (in fact, I am already doing so for my local builds). But for some reason, this fails in the Github actions:

C:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/14.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: D:/a/LibreVNA/Qt/6.8.3/mingw_64/lib/libQt6EntryPoint.a(qtentrypoint_win.cpp.obj):qtentrypoint_win.cpp:(.rdata$.refptr.__imp___argc[.refptr.__imp___argc]+0x0): undefined reference to `__imp___argc'

I don't know why this happens as the same code builds just fine against 6.8.3 on my machine. Not sure why there are these linker errors on the automatic builds. I also have not found a good way to debug this so far, because the only thing I can do is edit the yml build instructions, push another commit, wait 10 minutes for the build process and observe the output.