Add tooltip for milestone due date to issue and merge request lists

What does this MR do?

This MR adds tooltips for milestones' due dates listed in the issues and merge request lists.

Why was this MR needed?

It improves UX

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #18864 (closed)