API Compatibility Level is .NET 4

We *should* be able to put this back to the default as I do not believe that we need any of these features, and the project size will be smaller.

**Note that this is not the scripting runtime, which here must be .NET 4.**

See here. https://docs.microsoft.com/en-us/visualstudio/cross-platform/unity-scripting-upgrade?view=vs-2019

This requires testing.