Project cleanup

[wo80]

This pull request does some cleanup of the project files:

• Remove legacy .NET Framework 4.x targets from main project and add net6.0 target
• Remove unnecessary package references
• Update package references
• Remove AssemblyInfo.cs from main project
• Fix the unit test project, disabling one failing test (Github issue 14)

I marked this as draft, because before merging

1. I'd like to remove AssemblyInfo.cs from the test project, but there's an irritating copyright notice pointing to Hewlett-Packard. What's that about? Can it be removed?
2. I'd like to remove all stuff related to Nuget config. Packaging should be accessible from the dotnet command line, so no need to have a Nuget executable in the repository.
3. I'd like to change the folder structure, so that instead of having the projects in Source/Griffin.WebServer/ we'd just have a src/ folder.
4. I'd like to add a Github workflow, thus removing the Appveyor dependency

@jgauffin Please let me know which of the changes you approve and whether they should go into this pull request or a separate.

[wo80]

Btw, locally the tests are passing. So there seems to be some problem with the Appveyor setup.

[jgauffin]

Yep, I hadn't updated visual studio version. Have done that now and started a new build.

[wo80]

I addressed point 1) and 2) from the first post. If you want me to revert some of this, let me know.

The Appveyor build still fails:

Input file does not exist: src\Griffin.Framework\Griffin.Framework.sln

Obviously, Griffin.Framework.sln is nowhere in this repo, so still the problem must be the Appveyor setup.

EDIT: a possible solution to the current problems would be point 4), you'd just have to delete the Appveyor project.