Make Release builds Universal by default

Since the introduction of the support of Apple Silicon in 6.2, it is possible to make Universal builds for Qt apps. Still, by default, a build still contains artefacts only for one CPU architecture - the same as the host machine. The user can control this with the variables for cmake (https://doc.qt.io/qt-6/macos.html#architectures). and make the build "really" Universal

XCode proceeds with this in a different way according to "Update the Architecture List in Your Xcode Projects" in https://developer.apple.com/documentation/apple-silicon/building-a-universal-macos-binary . The current host CPU architecture is used for Debug builds, but Release builds are made Universal automatically.

The task is to match the behavior of XCode so that users have a consistent experience.

PS. Supporting this for 5.15 LTS is out of scope. The scope of the support of Apple Silicon on Qt 5.15 LTS is frozen with 5.15.9.