Show `merged date` in recently merged section
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=500107)
</details>
<!--IssueSummary end-->
In the recently merged section of the MR home page, having information about _when_ something was merged is more valuable than when it was created.
## Proposal
We should update to provide information on when something was merged.
issue