Fix default transparency level.

The macOS backend sets up a window with a transparency level of `None` but the managed code thinks it's `Transparent`, meaning that making the window transparent did nothing as it thinks it's already set.

Fixes #8419

src/Avalonia.Native/WindowImplBase.cs

@@ -501,7 +501,7 @@ namespace Avalonia.Native
             }
         }
 
-        public WindowTransparencyLevel TransparencyLevel { get; private set; } = WindowTransparencyLevel.Transparent;
+        public WindowTransparencyLevel TransparencyLevel { get; private set; } = WindowTransparencyLevel.None;
 
         public void SetFrameThemeVariant(PlatformThemeVariant themeVariant)
         {