config/types: ignore backport target branches which are `into` merge targets

When checking a "branch for release" MR, all of the history on the development branch is checked again with a target of the release branch. Since these MRs also backport to the target branch, set such branches up as "bases" so that their history is not considered "new" for the release branch. This allows the check to ignore the already-merged history and not recheck (potentially) thousands of commits which are already part of the relevant history anyways.