Upgrade projects to netcoreapp3.0

[natemcmaster]

Upgrade this project to netcoreapp3.0.

Marking as wip: because this causes test failures which I'm not 100% how to fix. It looks like the tests expect the samples to have a certain TFM. cc @pakrym can you help?

Part of dotnet/aspnetcore#3754

[natemcmaster]

JavaScriptSnippetTests are failing, presumably because they are trying to publish netcoreapp2.0 and 2.1 projects. Are these tests for backwards compatibility? Or should the also change to 3.0?

[pakrym]

@muratg do we want to continue running tests against AI adapter? I don't think they realize we do this and seem to be doing fine.

[muratg]

@pakrym what adapter? It's OK to disable tests to move forward with 3.0 port. What of our scenarios do these tests test?

I don't think they realize we do this and seem to be doing fine.

Let's make sure :)

[pakrym]

We are running tests against Microsoft.ApplicationInsights.AspNetCore. We don't own the scenario anymore we were just running them during transition time to make sure things go smooth.

We might also want to stop producing Microsoft.AspNetCore.ApplicationInsights.HostingStartup package.

[natemcmaster]

Tests pass now :)

[pakrym]

AI has it's new hosting startup here https://mseng.visualstudio.com/AppInsights/_git/StatusMonitor?path=%2FExtensions%2FExtension%2FAspNetCore.HostingStartup&version=GBmaster

And it's running a lot more tests than we did: https://github.com/Microsoft/ApplicationInsights-aspnetcore/tree/develop/test/Microsoft.ApplicationInsights.AspNetCore.Tests

[pakrym]

I think we should go forward with this to unblock other tasks.