Mirror dnceng/security-partners-dotnet release/* branches to dotnet-security-partners/dotnet

[lbussell]

Describe the Problem

Context: #3065

Now that we have an internal repo (security-partners-dotnet) with PR validation, we need a pipeline or CI that mirrors the contents of its release/* branches to our dotnet-security-partners repo.

Describe the Solution

Use the existing GitHub-to-AzDo code mirror as a reference: https://github.com/dotnet/arcade/blob/c5dd6a1da2e6d9b3423ab809fcda8af2927a408b/azure-pipelines-code-mirror.yml

[lbussell]

I have written a script that takes care of this as well as the release tagging here: https://gist.github.com/lbussell/2e3f445668e8c1f1f8eb83a8fd1ff4ad

We would need to adapt this into a pipeline that we can run once for each branch, or run manually once per month that mirrors and tags release/6.0.1xx and release/7.0.1xx.

[lbussell]

This is completed with #3187