Fix Issue #2251 .

As part of our depreciation of NetCoreApp2.1, we want to use FrameworkReferences in the AspNetCore project.
Due to some breaking changes, this is turning into a larger effort than expected. (#2412).
I'm breaking this into smaller tasks.
First, Adding NetCoreApp3.1 and Net5.0 to the AspNetCore project.
This change requires that I add the PublicApi files and will help guarantee that I'm not breaking any public apis.

Changes

• Add NetCoreApp3.1 and Net5.0 to the AspNetCore project.
  • Adding new PublicApi files.
  • In follow up PR, will change these frameworks to use FrameworkReference
• Keep NetStandard2.0.
• Revert changes to AspNetCore.Test project. Need this for backwards compat testing.

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

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