Add triage bot

Add triage bot to do a few things:

1. If typebug and no severity, ask author to add.
2. If HandbookOperations no priority, ask hb DRI to add.
3. Old issues: comment asking if still an issue, label as stale, auto-close if no activity.
4. Deal with stale MRs per below.

Problem to solve

MRs might remain unfinished, lost in todos, team members leaving, contributors not coming back, etc. This clutters the project and MR list over time.

Proposal

1. Comment into MR with a reminder.
2. Label MRs as stale
3. Auto-close MRs that do not have any activity for >6 months

The DevRel team has a triage bot which does similar operations: https://gitlab.com/gitlab-da/projects/devrel-bot/-/blob/master/.triage-policies.yml?ref_type=heads