move to dotnet 6

[haraldsteinlechner]

while the build script now is dotnet 6 (since 5 had a lot of problems in combination with fake etc) the application is still dotnet 5. so currently one needs dotnet 5 and dotnet 6 for development.
Steps to fix:

• switch all apps to dotnet 6
• change paket.dependencies accordingly
• check if all paths and deployment is unaffected
• consolidate readme.md