Fixing a memory leak when closing a window (#19699)

Added unsubscription from the X11Globals.WindowActivationTrackingModeChanged event in the Dispose method of the WindowActivationTrackingHelper class

src/Avalonia.X11/ActivityTrackingHelper.cs

@@ -81,6 +81,7 @@ internal class WindowActivationTrackingHelper : IDisposable
     public void Dispose()
     {
         _platform.Globals.NetActiveWindowPropertyChanged -= OnNetActiveWindowChanged;
+        _platform.Globals.WindowActivationTrackingModeChanged -= OnWindowActivationTrackingModeChanged;
     }
 
     public void OnNetWmStateChanged(IntPtr[] atoms)
@@ -88,4 +89,4 @@ internal class WindowActivationTrackingHelper : IDisposable
         if (Mode == X11Globals.WindowActivationTrackingMode._NET_WM_STATE_FOCUSED)
             SetActive(atoms.Contains(_platform.Info.Atoms._NET_WM_STATE_FOCUSED));
     }
-}
+}