- see #9923 for related discussion
- pin aspnet/EntityFrameworkCore dependencies
- chain aspnet/Extensions dependencies through aspnet/AspNetCore-Tooling instead
- run `darc` to grab the latest
- remove System.Text.Encodings.Web references
  - now included in runtime.*.Microsoft.NETCORE.App packages

nits:
- copy a few `ReferenceResolution.md changes from aspnet/Extensions version