SDK-ify csproj / sln files

[supermem613]

Let's upgrade our csproj / sln files to use the (newer) SDK style. This simplifies the files themselves and improves dependency management (nuget packages). This allows us to remove no longer needed build files and executables.

This change also adds to the About page the currently running .NET to help diagnose issues.

No actual runtime impact is expected. Just faster builds, more modern toolkit.

This work is pre-requisite for any future-facing work in #93.