NOTE: Can't use extension when only .NET 4.7 or below is installed

What happended

When we changed to include the Octokit.GraphQL repository as a submodule in #2127 and reference its projects (rather than referencing it as a NugGet package), this had the unintended consequence of changing which version of System.Net.Http any consuming projects referenced.

For users who have a new version of .NET installed this didn't break anything because there is an automatic binding direct from System.Net.Http, v4.1.1.0 to System.Net.Http, v4.0.0.0. Unfortunately users with an older version of .NET installed would experience the following exception:

System.MissingMethodException: Method not found:
'System.Net.Http.HttpMessageHandler Octokit.Internal.HttpMessageHandlerFactory.CreateDefault()'.
at GitHub.Infrastructure.ExportedHttpClient..ctor()

What this PR does

This PR ensures that we reference the System.Net.Http, v4.0.0.0 assembly by changing the Octokit.GraphQL projects to compile against .NET 4.6 rather than .NET Standard 1.1 as they were before. This is a simple fix that should allow us to get a patch release out ASAP.

• Add test to check that GitHub.* assemblies only reference System.Net.Http, v4.0.0.0
• Compile against .NET 4.6 version of Octokit.GraphQL and Octokit.GraphQL.Core

How to test

• Check that .vsix doesn't contain a System.Net.Http assembly.

Here is a screenshot of the .vsix contents from this PR:

More detail

On recent versions of .NET there is an automatic redirect from System.Net.Http, v4.1.1.0 to v4.1.1.0. Here is a binding log generated by fuslogvw:

Attempting to load System.Net.Http, v4.1.1.0 inside a docker image build from microsoft/dotnet-framework:4.6.2-runtime fails.

Step 20/23 : FROM microsoft/dotnet-framework:4.6.2-runtime AS runtime
 ---> 91af6975d04b
Step 21/23 : WORKDIR /app
 ---> Using cache
 ---> c3fc388c3cf6
Step 22/23 : COPY --from=publish /app/dotnetapp/out ./
 ---> 405821dc4a0d
Step 23/23 : ENTRYPOINT ["dotnetapp.exe"]
 ---> Running in 00c145e35a59
Removing intermediate container 00c145e35a59
 ---> 9f064e8ae099
Successfully built 9f064e8ae099
Successfully tagged foo3:latest
C:\Source\github.com\Microsoft\dotnet-framework-docker\samples\dotnetapp [master ≡ +0 ~4 -0 !]> docker run foo3
Attempting to load: System.Net.Http, Version=4.1.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a

Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly 'System.Net.Http, Version=4.1.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.

I don't know when exactly this redirect was put in place, but it was somewhere between .NET 4.6.2 and .NET 4.7.2.

Fixes #2190
Fixes #2188
Fixes #2186
Fixes #2185
Fixes #2195