Update Ookii.Dialogs.Wpf to support .NET 7

[augustoproiete]

.NET 7 has been released 🎉

[cocoon]

just as Info, .NET 8 got an own one now :)

https://devblogs.microsoft.com/dotnet/announcing-dotnet-8-preview-7/?WT.mc_id=dotnet-35129-website#openfolderdialog-in-wpf

dotnet/wpf#7244