Override the line-height for the specific markdown editor

What does this MR do and why?

For #571065 (closed)

Override the 1rem line-height for the review summary panel-specific markdown editor with the standard root 1.5 line-height.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Have an MR with changes
2. Leave a comment and "add to review" instead of directly submitting the comment
3. Click the "Your review" button
4. Click into the "Add optional summary content..." text box
5. Add multiple lines of text in the resulting text area
6. Note the line-heights in the textarea

MR acceptance checklist

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