Upgrade Package to dotnet 6

[d-barker]

Updated the project to dotnet 6.0:

1. Removed support for netcoreapp3.1 instead going for net6.0 and netstandard2.1
2. I fixed the unit tests by removing references to the error checking with {user} embedded in the error, for some reason the user id is no longer sent in the error message. I also updated the error checking code for error messages to be a little more relaxed and do a contains rather than explicit equals.
3. There are two sign in tests that were failing, In order to fix, a claim was added to the user in both external and non external tests. Possibly something that used to add a default claim no longer adds this?
4. Updated the Yaml file to include dotnet 6 instead of 3.1, this has not been tested.