Introduce a new 'DefaultLabelUponClosing' processor

What does this MR do and why?

This adds typemaintenance to security and documentation issues & MRs that are closed/merged without a type label already set.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #769 (closed).