feat(css): add gl-mt-n3 utility

What does this MR do?

Adds the gl-mt-n3 utility class. Adds a negative top margin with spacing size 3.

For use in gitlab!39822 (merged).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [n/a] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [n/a] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [n/a] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • [n/a] GitLab: mr_url
  • [n/a] Customers Portal: mr_url
  • [n/a] Status Page: mr_url
• [n/a] Added the ~"component:*" label(s) if applicable.