Improve prompt-stale-mrs.yml message

What does this MR do and why?

Describe in detail what your merge request does and why.

Revises the wording of the stale MR prompt

More revisions coming with #908 (closed)

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