Add two policies to ping inactive reviewers on community MRs, and to ping MR coaches on community MRs ready for review

What does this MR do and why?

This adds two policies to:

1. Ping inactive reviewers on community MRs ready for review to either review or reassign to someone else.
   • Conditions:
     • MR has Community contribution and workflowready for review
     • MR doesn't have automation:reviewers-reminded
     • No activity on the MR in the last 7 days
     • MR has reviewer(s) assigned
2. Ping MR coaches on community MRs ready for review.
   • Conditions:
     • MR has Community contribution and workflowready for review
     • MR doesn't have automation:reviewers-reminded
     • No activity on the MR in the last day (i.e. after 1 day of inactivity, ping an MR coach)
     • MR has no reviewers assigned

In both policies, we hint to add workflowin dev if some work from the author is actually required.

If the reviewer doesn't take any action, the policy won't fire again, unless automation:reviewers-reminded is removed.

These policies should ensure that the ball is rolling when a Community contribution MR is under a GitLab team member responsibility.

Following is a sample message of a MR coach ping that would happen on https://gitlab.com/gitlab-org/build/CNG/-/merge_requests/989:

@mayra-cabrera, this ~"Community contribution" is ready for review.

- Do you have capacity and domain expertise to review this? We are mindful of your time, so if you are not
  able to take this on, please re-assign to one or more other reviewers.
- Add the ~"workflow::in dev" label if the merge request needs action from the author.

Following is a sample message of a MR coach ping that would happen on https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1834:

@ggeorgiev_gitlab, this ~"Community contribution" was recently assigned to you for review.

- Do you still have capacity to review this? We are mindful of your time, so if you are not
  able to take this on, please re-assign to one or more other reviewers.
- Add the ~"workflow::in dev" label if the merge request needs action from the author.
/label ~"automation:reviewers-reminded"

Closes #897 (closed).

Expected impact & dry-runs

Based on https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/2655478858, it would ping:

• MR coaches on 20 MRs
• Currently assigned reviewers on 16 MRs

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