Make sure all new packages have been mirrored to dotnet-public once per year

Every time we bump major version, we get a set of "new" packages, since packages like `Microsoft.DotNet.Web.ItemTemplates.7.0` have the Major version in their name. This means someone needs to manually mirror these packages to dotnet-public so that further versions on nuget.org get auto-mirrored. We should have an automatic process to pick these up (along with new OOB package that might come along in a major release) so that we don't have to remember to do this manually every year.

Start by reviewing the existing manual process for mirroring new packages to dotnet-public and how packages from major releases are identified. Define an automatic yearly or major-version workflow that also finds new out-of-band packages, then verify that later nuget.org versions are mirrored without manual setup.

Written by the indexing model from the issue text.