This is the first phase of the work for dotnet/source-build#4141. Before repos can migrate off their reference to the vulnerable version, we need to have the non-vulnerable version available in SBRP.

Adds the following packages:

• NuGet.Frameworks.6.7.1
• NuGet.Common.6.7.1
• NuGet.Configuration.6.7.1
• NuGet.Versioning.6.7.1
• NuGet.Packaging.6.7.1
• NuGet.Protocol.6.7.1
• NuGet.Credentials.6.7.1
• NuGet.LibraryModel.6.7.1
• NuGet.DependencyResolver.Core.6.7.1
• NuGet.ProjectModel.6.7.1
• NuGet.Commands.6.7.1
• NuGet.Frameworks.6.8.1
• NuGet.Common.6.8.1
• NuGet.Configuration.6.8.1
• NuGet.Versioning.6.8.1
• NuGet.Packaging.6.8.1
• NuGet.Protocol.6.8.1
• NuGet.Credentials.6.8.1
• NuGet.LibraryModel.6.8.1
• NuGet.DependencyResolver.Core.6.8.1
• NuGet.ProjectModel.6.8.1
• NuGet.Commands.6.8.1

Deletes the following packages because they are unused:

• NuGet.Frameworks.6.6.1
• NuGet.Common.6.6.1
• NuGet.Configuration.6.6.1
• NuGet.Versioning.6.6.1
• NuGet.Packaging.6.6.1
• NuGet.Protocol.6.6.1
• NuGet.Credentials.6.6.1
• NuGet.LibraryModel.6.6.1
• NuGet.DependencyResolver.Core.6.6.1
• NuGet.ProjectModel.6.6.1
• NuGet.Commands.6.6.1