Fix promoting labels and milestones copy text

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/issues/44208 that includes specific copy text for promoting milestones

Screenshots

Milestones	Labels

Flash messages

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/44208