Add a daily report for untriaged 'Community contribution' MRs

This adds a daily report for untriaged 'Community contribution' MRs, in order to make sure all Community contribution MRs have a stage and group label assigned.

That would then allow to ping the relevant EM/PM/engineers on the MRs automatically.

Dry-run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/714139385

Handbook:

• MR: gitlab-com/www-gitlab-com!60903 (merged)
• Review App: http://localhost:4567/handbook/engineering/quality/triage-operations/#newly-created-community-contribution-merge-requests-requiring-first-triage

Part of #271 (comment 401842156).