Fixes: dotnet#22973
Context: https://github.com/danielancines/memory-leak-dotnet8-maui

After reviewing the above sample app, I found that the
`MauiWinUIWindow` class was not ever going away. I could see it being
held by `NavigationRootManager`:

* `Microsoft.Maui.MauiWinUIWindow` ->
  * `Microsoft.Maui.Platform.NavigationRootManager` ->
    * `Windows.Foundation.TypedEventHandler<Microsoft.UI.Xaml.Controls.NavigationView, Microsoft.UI.Xaml.Controls.NavigationViewBackRequestedEventArgs>`

After some trial and error, I found that simply changing
`NavigationRootManager`'s `_platformWindow` field a `WeakReference`
"untangles" things, and the sample no longer leaks `MauiWinUIWindow`
instances.

Unfortunately, I was not able to write a passing test for this issue.
No matter what I tried, the platform `Window` is kept alive by a
"strong handle". But taking a snapshot after the test ends, it goes
away?

I still thing the test is useful, as it verifies the other objects.