Upgrade to .NET 6

[ErikSchierboom]

With .NET 6 released, we should be updating all exercises to .NET 6. This will allow students to use C# 10.

Note that we should make sure that the csharp-test-runner, csharp-representer and csharp-analyzer all work. We likely want to update them to .NET 6 too.

[andrerfcsantos]

Complete C# noob here, but this might help? https://dotnet.microsoft.com/platform/upgrade-assistant

[ErikSchierboom]

Yeah that's a nice link. Upgrading is actually really simple though, which we can do with just a search/replace :)