Error with --list-extensions listing for extension that target .NET Standard

[ChrisMaddock]

From nunit/teamcity-event-listener#75 (comment)

The TeamCity extension which targets .NET Standard 2.0 was displayed in the --list-extensions output as targeting .NET 4.0.

I presume this was because the console was running on .NET 4.7ish, and the console is instead (incorrectly) displaying the extensions runtime. So two issues:

1. We should be displaying an extensions target framework, rather than the current runtime framework.
2. We should look at when .NET 4.0 was displayed instead of .NET 4.7. I suspect that an extension which targets .NET 4.7 would also display as .NET 4.0, which is incorrect.