MacOS versions and RsyncUI

1 December 2023 - next release later in December 2023 will only support macOS Sonoma. By every new release of SwiftUI and macOS there are several new features which also are beneficial for RsyncUI. The result is the macOS Sonoma version of RsyncUI includes enhancements not in the other versions. And it is difficult to maintain two release of RsyncUI. There are too many #avaliable statements in code which makes the code more vulnerable for bugs.