[Development] Raising minimum CMake version to 3.16 for Qt6

[Kai Köhne]

> -----Original Message-----
> I'm not really remembering at the moment. The development hosts were
> already set to GCC9 for 6.0. So maybe GCC8 works, but we're not testing it
> anywhere (and that makes it basically unsupported). 

Well, we're still building things on Windows with gcc 8.1. See e.g.

https://testresults.qt.io/logs/qt/qt5/de93708ae5d95f77df85a130b5de546b2d46c1fe/WindowsWindows_10x86_64WindowsWindows_10x86_64Mingwqtci-windows-10-x86_64-52-d843faInsignificantTests_Sccache/d76ebc5a537502304acc418e6270be882de2a953/build_1620360977/log.txt.gz

> agent:2021/05/07 04:24:49 build.go:379: Building for: win32-g++ (x86_64, CPU features: cx16 mmx sse sse2 sse3)
> agent:2021/05/07 04:24:49 build.go:379: Compiler: gcc 8.1.0

So it might have regressed in Unix specific parts, but overall we still support gcc 8.1.

Regards

Kai