Remove support for v2 publishing in the templates (#8325)

Documentation/CorePackages/Publishing.md

@@ -230,7 +230,7 @@ In order to use the new publishing mechanism, the easiest way to start is by tur
 
     | Name                                    | Type     | Description                                                                                          |Default Value |
     | --------------------------------------- | -------- | -----------------------------------------------------------------------------------------------------|----- |
-    | publishingInfraVersion                  | int      | Publishing infrastructure version - Use 3 for latest publishing infra. Accepted values are 2 / 3.                               | 2    |
+    | publishingInfraVersion                  | int      | Publishing infrastructure version - Use 3 for latest publishing infra. Accepted values are 3 (.NET 5.0+) and 2 (.NET 3.1).                               | 3    |
     | enableSourceLinkValidation              | bool     | Run SourceLink validation during the post-build stage.                                               | false |
     | enableSigningValidation                 | bool     | Run signing validation during the post-build stage.                                                  | true |
     | enableNugetValidation                   | bool     | Run NuGet package validation tool during the post build stage.                                       | true |
@@ -250,7 +250,7 @@ In order to use the new publishing mechanism, the easiest way to start is by tur
     * [Arcade-Validation](https://github.com/dotnet/arcade-validation/blob/master/azure-pipelines.yml)
     * [Arcade-Services](https://github.com/dotnet/arcade-services/blob/master/azure-pipelines.yml)
 
-1. Create or update eng/Publishing.props, adding the following MSBuild property:
+2. Create or update eng/Publishing.props, adding the following MSBuild property:
     ```XML
         <PublishingVersion>3</PublishingVersion>
     ```