Now that Microsoft has officially released .NET 6,
I have upgraded the C# version of Astronomy Engine to use it.
No source code changes were needed. I just bumped the
version number in the project files, and targeted .NET 6
in the GitHub Actions continuous integration tests.
Fixed some obsolete wording in generate/README.md.
I'm checking in a broken build on purpose in this topic branch
so that I can move the work over to my Windows machine and use
Visual Studio to help with formatting XML comments.