Update dotnet new for .NET Core 3.0/3.1 release (#14356)

* initial commit

* save changes

* save updates

* more updates

* more updates

* more edits

* more edits

* break down syntax block

* argument formatting

* fix broken tables

* add last separator

* remove note about 1.1

* further changes

* more formatting

* grammar

* additional info

* Apply suggestions from peer review

Co-Authored-By: Tom Dykstra <tdykstra@microsoft.com>

* feedback

* more feedback

* link to default C# version

Co-authored-by: Tom Dykstra <tdykstra@microsoft.com>

Author: mairaw