How to pick WindowGroup or Window depending on the OS in SwiftUI?

Question

In my macOS GUI app I need to show only one window. If the user closes it, the app should also close.

So I can do it as such:

@main
struct MyApp: App {
    var body: some Scene
    {
        Window("my title",
               id: "idMyWindow")
        {
            ContentView()
            .onAppear
            {
                //Code that runs when app is shown
            }
        }
    }    
}

But I also need to have my app run on macOS v.12. When I change that the compiler tells me that Window is not available. So I need to revert back to WindowGroup for an older OS.

So I tried this instead:

@main
struct MyApp: App {
    var body: some Scene
    {
        if #available(macOS 13.0, *)
        {
            Window("my title",
               id: "idMyWindow")
        }
        else
        {
            WindowGroup
        }
        {
            ContentView()
            .onAppear
            {
                //Code that runs when app is shown
            }
        }
    }    
}

But evidently that is not how you use #available in that case.

Any idea how shall I change it to make it work?