WindowBase 4.0/5.0 mismatch when using recent versions of WebView 2 Control

Steps to repo:

* Create a new WinForms App, target anything between .NET 8 and .NET 10 Preview.
* Add the _latest_ WebView2 package to you project:

```



```

* Recompile and observe the build output/error window.


This is probably merely an annoyance, but a big one, especially for those customers who warn as error. Googling this, it also seems to annoy _quite_ some folks at this point, and also some of them were allegedly even blocked when they had problems to pack or create installers. (That, I could no repo though.)

So. The WindowBase 4.0 we're having here, what do we need it for exactly (especially in the WinForms case, when we're not using `ElementHost`)?

It's a type-forwarder, basically, is it not?

If that's the case, couldn't we just for .NET 8+ have that in the same version 5.0? Or do we need it in version 4.0 for scenarios, we're we'd add references to NETFX assemblies?

@dsplaisted, @ericstj and @JeremyKuhne FYI.