Have cherry-pick bot test for public lts-* branches before falling back to tqtc/ private repos

Some repos may need to remain fully open-source on LTS branches, even though the given LTS branch is tqtc private in other repos. In these cases, the public-only LTS branches should be prefixed with "lts-", and the cherry-pick bot should prefer to use the public LTS branch if available and open for new changes. If no public branch exists or is closed, the current fallback logic of trying in tqtc/ repos should apply.