Add UI tip for merge trains

What does this MR do?

This adds message to the stage dropdowns that indicates that a pipeline can't be retried when the selected merge strategy is merge trains

#285529 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content