remove AspNetCore net461 (prep FrameworkReference)

[TimothyMothra]

Fix Issue #2251.

We're aligning the AspNetCore project with NetCoreApp3.1, Net5.0, and Net6.0.
Formerly, NetCoreApp2.1 was compatible with targeting Net Framework. This is no longer available in the newer framework versions and we're removing it from this project.

Changes

• Removing Net461 from AspNetCore project.
• Removing preprocesosrs.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

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

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

Notes for reviewers:

• We support comment build triggers
  • /AzurePipelines run will queue all builds
  • /AzurePipelines run <pipeline-name> will queue a specific build

[cijothomas]

LGTM. To make sure we are not missing anything need another look from @rajkumar-rangaraj @vishweshbankwar @reyang