Update untriaged-community-merge-requests.yml to include assigning a reviewer

What does this MR do and why?

Update untriaged-community-merge-requests.yml to include assigning a reviewer

• our documentation states that a community contribution should be assigned a reviewer by the engineering manager, but often times a community contribution is for an engineering-wide initiative that can be reviewed by anyone
• this update suggests that an MR coach should assign a reviewer for these types of merge requests

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

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