Allow Deliverable on expired milestones

When creating or changing a new triage policy or triage package:

• Consider adding documentation to the handbook pages for Triage Operations: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/handbook/engineering/quality/triage-operations/index.html.md
• Consider announcing to the appropriate Slack channels or week-in-review

We don't seem to mention expired milestones on https://about.gitlab.com/handbook/engineering/workflow/#milestone-cleanup so I guess it's fine not updating it. Thoughts? Since it's never mentioned, it's not fully clear to me though. Maybe we should also mention that Deliverable will be kept on expired milestones?

Closes #418 (closed)