Ask for severity label, priority label, and milestone on ~infradev issues

This adds a new hygiene policy that will run daily to ask for severity label, priority label, abnd milestone on issues that:

• Miss a severity label, priority label, or milestone.
• Don't have the automation:infradev-missing-labels label, or the label was set more than 2 weeks ago and on a day multiple of 2 weeks (so the bot will post only every two weeks).

Dry-run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/1212238642

Open questions

• The bot will leave a message every day, starting two weeks after the automation:infradev-missing-labels label is first set.
  • I don't think it's good but I'm not sure what we want to do here?
  • Maybe we could ping once every two weeks after the label is first set. Something like (label_added_at % 2_weeks).to_date == Date.today.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations => gitlab-com/www-gitlab-com!80823 (diffs)
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review

Closes #673 (closed)