Add a warning when pipeline was created too long ago

Context

• Follow-up of gitlab-org/quality/engineering-productivity/master-broken-incidents#379 (closed).
• Closes gitlab-org/gitlab#386752 (closed)

What does this MR do and why?

• Display a warning to the user that merged the MR if the latest pipeline was created more than 6 hours ago.

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