Update Microsoft.WindowsAppSDK version in WinAppSdk.props

[Arlodotexe]

This PR fixes a package downgrade warning when the latest Win2d packages are used:

"C:\a\Windows\Windows\CommunityToolkit.AllComponents.sln" (Restore target) (1) ->
       (Restore target) -> 
         C:\a\Windows\Windows\components\ImageCropper\samples\ImageCropper.Samples.csproj : error NU1605: Warning As Error: Detected package downgrade: Microsoft.WindowsAppSDK from 1.6.240923002 to 1.6.240829007. Reference the package directly from the project to select a different version.  [C:\a\Windows\Windows\CommunityToolkit.AllComponents.sln]
       C:\a\Windows\Windows\components\ImageCropper\samples\ImageCropper.Samples.csproj : error NU1605:  ImageCropperExperiment.Samples -> CommunityToolkit.WinUI.Controls.ImageCropper -> Microsoft.Graphics.Win2D 1.3.0 -> Microsoft.WindowsAppSDK (>= 1.6.240923002)  [C:\a\Windows\Windows\CommunityToolkit.AllComponents.sln]
       C:\a\Windows\Windows\components\ImageCropper\samples\ImageCropper.Samples.csproj : error NU1605:  ImageCropperExperiment.Samples -> Microsoft.WindowsAppSDK (>= 1.6.240829007) [C:\a\Windows\Windows\CommunityToolkit.AllComponents.sln]

See also https://github.com/CommunityToolkit/Windows/actions/runs/11523058494/job/32088060877#step:11:3515