Add 'closed_at' attribute to Issues API

What does this MR do?

This MR adds the closed_at attribute to the issues API endpoints.

Why was this MR needed?

It makes our API a bit greater. =)

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 #5935 (closed)