Don't try to generate incrementals for versions which don't set incremental_from

With #41247 (closed) we now define two versions in update-responses config, and the script will look at all versions when generating incrementals. This will fail if the legacy version is not present in the signed directory. However we don't need to generate incrementals for the legacy version (they are generated from the legacy branch). Instead, the update_responses script should skip versions which don't define incremental_from.