Fix C# templates on older frameworks #57
Description
iInstead of overcomplicating our C# template setup with having LangVersion as settable parameter too, I leave only Framework as settable, and add a LangVersion=12.0 for netstandard and net5.0 where we need it for clean modern code (implicit usings, file-scoped namespaces)
Otherwise I can leave LangVersion as default (tied to the Framework automatically) as .NET 6+ have all we need