Add closedAt to issue graphql fragment

What does this MR do and why?

The closedAt field was not being queried in graphQL calls for issues, which prevented FE logic from using this data to determine whether or not to show a danger style for due dates on closed issues in boards.

Screenshots or screen recordings

Before	After	What changed?
		Because closedAt data is being queried for issues, the FE logic already in place is able to correctly determine that it should not show the due dates on closed issues in red.

How to set up and validate locally

1. Check out this branch
2. Visit a project issue board that contains closed and open issues
3. assign a due date in the PAST to an OPEN issue

• expected: you will see that the due date shows in red

4. assign a due date in the PAST to a CLOSED issue

• expected: you will see that the due date shows in grey

5. Drag the overdue issue from step 3 to the closed column

• expected: the due date will go from being red to being grey once the issue is closed

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.