KColorSchemeWatcherMac: do not use new NSAppearance API on older macOS versions

On macOS versions older than Big Sur (11.0) the
`NSAppearance.currentDrawingAppearance` property does not exist, and
trying to use it will crash the app at runtime.

This commit makes sure the new property is used on Big Sur or newer
macOS releases, and uses the deprecated `NSAppearance.currentAppearance`
property on Mojave (10.14) or newer.  As the light/dark mode switch was
introduced in Mojave, running the app on older releases will always
prefer the light mode.

Tested on macOS 10.15.5 (x86_64) and 13.1 (arm64)

BUG: 463752