refactor(dev-infra): expose logic for dealing with LTS branches (#38656)
refactor(dev-infra): expose logic for dealing with LTS branches (#38656)

Exposes logic for dealing with LTS branches, so that the release
tool can re-use it for cutting LTS patch releases.

Eventually, we can move all of this logic to a more dedicated
folder instead of having it inside the merge folder.

PR Close #38656

angular/dev-infra,angular/dev-infra,angular/dev-infra,angular/dev-infra