Add actual time that the MR was merged to merged MR list

What does this MR do and why?

For #383514 (closed)

Adds the merged_at timestamp value to a tooltip wrapped around the Merged badge.

The tooltip format matches the updated value just below in the second row of the MR item.

Screenshots or screen recordings

Before	After
(There is effectively no "before"; the date/tooltip are not available before this change)

How to set up and validate locally

1. Have at least 1 merged MR
2. Visit the Merge Requests list page
3. Change to the Merged tab
4. Hover over the blue "Merged" badge

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.