Implement automated dependency flow using darc

[natemcmaster]

See https://github.com/dotnet/arcade/blob/master/Documentation/DependencyFlowOnboarding.md

We should implement automated dependency flow. This would replace using dotnet/versions and this script: https://github.com/aspnet/AspNetCore/blob/master/scripts/UpdateDependenciesCoreFx.ps1.

Matching bug on the dotnet/arcade side: dotnet/arcade#673

[natemcmaster]

We implemented dependency flow on Extensions in dotnet/extensions#626. We'll probably be able to copy this approach on this repo while we make progress on converting to the Arcade SDK.

[natemcmaster]

To be clear, Arcade SDK conversion will come later and likely require big changes to aspnetcore. We'll do dependency flow first to unblock our partner teams.