Update all 2.0 samples to use M.A.App, aka Microsoft.AspNetCore.App

[Rick-Anderson]

This is the master issue.

[guardrex]

Recommendation: Use #5495 because:

• Sample Update Tracking Issue - complete list #5495 has all of the samples listed (e.g., project.json samples slipped through the cracks on prior update attempts).
• Authors won't trip over each other trying to get all of the samples updated. I marked the ones that I'd like to handle, if that's ok. I built most of the ones that I self-assigned.
• We can coordinate what is to be updated that goes beyond just the "App" package (e.g., static asset management, BuildWebHost to CreateWebHostBuilder, naming, etc.).