Remove Backdrop fallback workaround

[dotMorten]

In 1.2, WinAppSDK is going to provide the fallback for background when mica/acrylic isn't supported: https://github.com/microsoft/WindowsAppSDK/issues/2637

This means WinUIEx can remove the workaround for that once 1.2 ships: https://github.com/dotMorten/WinUIEx/blob/ce3b8d6d270b8124c4f16acc00b694fbc3d2dd9e/src/WinUIEx/WindowManager.Backdrop.cs#L73-L105

[rocksdanister]

This means WinUIEx can remove the workaround for that once 1.2 ships:

Will you check the running WinAppSDK version the app is using.. what if its lower than 1.2?

[dotMorten]

what if its lower than 1.2?

Once 1.2 ships WinUIEx will require 1.2

[dotMorten]

Completed in #e1a07a58dc06b648e3fc59dd2ebb7dd3f25c6fdf