Update Dotnet Sample to Include Debugging / move to dotnet 3.1

crw 1.2 did not include a debugger for .net. In Che 7, we do have a debugger, but that would require us to update the base image to .net 2.2 and to update the example.

We could either use the upstream example or update the one we have in crw 1.2

• https://github.com/eclipse/che-devfile-registry/blob/master/devfiles/dotnet/devfile.yaml#L21 (.net 2.2)
  vs.
• https://github.com/redhat-developer/codeready-workspaces/blob/master/dependencies/che-devfile-registry/devfiles/dotnet/devfile.yaml (.net 2.1)