Implements UpdateKeyStateFromModifiers helper to detect Ctrl/Shift/Alt states from mouse events, enabling enhanced interactions like Ctrl+MouseWheel zoom.

Also upgrades Windows App SDK to 1.7 and lowers target framework to net8.0-windows10.0.19041.0 due to v1.5 runtime issues in my environment (can be reverted if needed).

Fixes #4967

Additional Notes

For future KeyDown/KeyUp event handling from SKXamlCanvas in WinUIPlot.cs, the CreateRenderTarget() method would need focus-related properties:

private static SKXamlCanvas CreateRenderTarget()
{
   return new SKXamlCanvas
   {
       // ... (existing VerticalAlignment, HorizontalAlignment, Background) ...
       
       IsTabStop = true,                    // Enables the control to receive focus
       AllowFocusOnInteraction = true       // Allows focus on mouse interaction
   };
}

This PR focuses on mouse-based modifier detection using PointerRoutedEventArgs, which doesn't require focus.