Automate deletion of inactive private repos

Private repos will be enabled to allow fixing vulnerabtilies behind closed doors. This is the only use case we want to have private repos. It means that private repos are only temporary and should be cleaned up once a fix is published. To keep disk space usage under control, we want to ensure that private repos that are not inactive get garbage collected after some time.

It can be done via UI https://docs.gitlab.com/ee/administration/inactive_project_deletion.html. But it seems that it's an all or nothing rule. We want to be delete inactive private projects, not public ones. We may need to use the API then.