Adds netstandard2.0 target for dependencycollector.

[cijothomas]

UnitTests converted to target 2.1 instead of 1.1. Unit tests were blind about 2.0 before this, and now they will be blind about 1.1 instead. .NET Core 1.1 is already officially out of support, but SDK still has target for 1.1

We may chose to have 1.1 and 2.1 tests, instead of 2.1 alone. Fix Issue #1212 .

• [ ] I ran Unit Tests locally.

For significant contributions please make sure you have completed the following items:

• [ ] Changes in public surface reviewed

• [ ] Design discussion issue #

• [ ] CHANGELOG.md updated with one line description of the fix, and a link to the original issue.

• [ ] The PR will trigger build, unit tests, and functional tests automatically. If your PR was submitted from fork - mention one of committers to initiate the build for you. If you want to to re-run the build/tests, the easiest way is to simply Close and Re-Open this same PR. (Just click 'close pull request' followed by 'open pull request' buttons at the bottom of the PR)

• Please follow [these] (https://github.com/Microsoft/ApplicationInsights-dotnet-server/blob/develop/CONTRIBUTING.md) instructions to build and test locally.

[TimothyMothra]

I understand the reason for switching from 1.1 to 2.1, but this makes me just a little bit nervous. I would feel better if we added 2.1 now, and removed 1.1 at a later date.

Other than that, this looks good to me. :)

[cijothomas]

@MS-TimothyMothra we were running without 2.0 tests until now :( Since 1.1 is deprecated officially, we can live without testing 1.1. Anyway if all insist for 1.1 tests, I am happy to get it added. (I am planning to soon add 3.0 tests, so want to get rid of 1.1 sooner...)

[cijothomas]

Plan is to anyway cut support for 1.1 by the time 3.0 releases. So it is okay to shift unit tests to target 2.1 instead of 1.1.