Provide a way to get an actual OS version

I'm using QSysInfo::productVersion. For me, it returns 10.0 under Windows 11, and 10.16 under macOS Big Sur, macOS Monterey.

It's NOT a Qt bug. It's the way these OSes behaves. However, it's still possible to detect actual versions.

Windows: one have to use RtlGetVersion + some additional tricks.
https://stackoverflow.com/questions/69373447/is-there-an-official-way-to-detect-windows-11.

macOS: version can be read from this file: /System/Library/CoreServices/.SystemVersionPlatform.plist
https://stackoverflow.com/questions/75413538/troubles-getting-an-actual-macos-version

So, I suggest something like QSysInfo::nonCompatProductVersion. It should try to do its best to detect an actual version. But it should be explicitly stated in the docs, that there is no guarantee for it to work 100% accurate.