Polish phrasing around "merge request"

What does this MR do and why?

Applies small polishes to the language in these messages:

• triage/processor/breaking_change_comment.rb
• triage/processor/doc_community_contribution.rb
• triage/processor/hackathon_label.rb
• triage/processor/merge_request_help.rb
• triage/processor/new_pipeline_on_approval.rb
• triage/processor/plan_ux_mr_review_support.rb
• triage/processor/ux_community_contribution.rb

Updates the corresponding tests.

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