Emphasize irreversibility in warning message when promoting labels and milestones

What does this MR do?

Changes the copy text for promoting the milestone and labels

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #40920 (closed)