Add gl-sm-mt-6 utility class

Summary

We should add a gl-sm-mt-6 utility CSS class, we currently have gl-sm-mt-5 and gl-sm-mt-7 available but now have a use case for 6.

Improvements

• Once the utility is available in gitlab we can replace the custom css in gitlab!97387 (diffs)

Involved components

Optional: Intended side effects

• Additional class available for use