Resolve "Add more whitespace to Merge Topics Modal"

What does this MR do and why?

Replaces div with gl-form-group to apply automatic spacings.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

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

1. As an admin, go to http://gdk.test:3000/admin/topics
2. If there are no topics, create two topics
3. Open the modal "Merge topics"
4. Select two different topics

Closes #477254