feat(css): Add margin top auto

What does this MR do?

gl-mt-auto is needed by the top nav redesign.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• ~~[ ] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.~~
• [ ] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [ ] 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:
• [ ] Added the ~"component:*" label(s) if applicable.