Add milestone issues display limit

What does this MR do?

Limit the number of Issues displayed on the Milestones page and adds a warning.

related to #39453 (closed)

Screenshots(stubbed the issues limit to 6)

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