feat(ui): Add icon to Merge Request popover

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What does this MR do and why?

Adds an icon to the Merge Request (MR) popover

References #524294 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up GitLab locally and start the development environment.
2. Go to a repository in GitLab.
3. Find a comment mentioning a Merge Request (e.g., !9).
4. Hover over the !9 to trigger the popover.
5. Verify that the icon appears correctly within the popover.