Add label backlog to exempt labels. We will use label backlog to collect bugs/features that we want to consider for a future release. Thus we do not want to auto-close those issues.
With this, and the already activated github-app, the stale robot will
mark and subsequently close issues as specified in the config.
Currently we mark after 21 days, close after 7 more days, and ignore
issues with an assigned priority or milestone.