Update MR and Issue template for label migrations

What does this MR do and why?

This MR updates the issue and MR templates for label change and label migration requests. The goal is to make the label migration documentation visible to team members during submission of the issue and also when submitting the one-off migration MRs.

Expected impact & dry-runs

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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