Reduction of the build matrix

[hmaarrfk]

Comment:

@traversaro i wanted to have the conversation here to see if there was any blocker on your end.

FFMPEG 7.0 has been pretty good to me (7.1 seems problematic but I think I got through it....)

Qt6 has been good to me.

Thus I don't see the need to build out FFMPEG 6 and Qt6 anymore.

let me know if you see any reason to keep them around.

[traversaro]

I agree in dropping both (and look into closing the ffmpeg6 migration). Unfortunately I still need to build some of my packages with qt5, but as qt6 and qt5 are side-by-side side installable compiling opencv just with qt6 is perfectly fine for me.