Correct the vertical margin for the top nav's "Sign in / Register" button

What does this MR do?

Correct the vertical margin for the top nav's "Sign in / Register" button

Before

After

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

Closes #53673 (closed)