Restrict access to release branches for LTS Refresh. BUG=b/571158181 Change-Id: I88cfaef61d30f7d8f8c2b28c861da5f2255d7b9a Reviewed-on: https://cos-review.googlesource.com/c/All-Projects/+/223487 Reviewed-by: Kevin Berry <kpberry@google.com> Tested-by: Derek Taylor <ddtaylor@google.com>