Fix vertical alignment of text in diffs

What does this MR do?

This fixes the vertical alignment of text in diffs.

Notes:

• The vertical alignment was off for not only line numbers, but also the line text.
• This issue was present throughout GitLab. It affected diffs in the commit page and not just the MR page.

Screenshot

Before	After

What are the relevant issue numbers?

Closes #48902 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides