Handle more master to maining (#734)
Handle more master to maining (#734)

* Handle more master to maining

- in midst of renaming default branch of dotnet/aspnetcore-tooling
- ef6 'master' branch no longer exists, should not be target of merge PRs
- aspnet/aspnetcore-internal doesn't have an internal mirror

* Update Maestro/subscriptions.json
dotnet/versions