feat: add gl-mx-n6 utility mixin

What does this MR do?

It adds gl-mx-n6 utility mixin to spacing.scss, the class will be used in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/3490#a14853042e30d65797b6098721fd569ba9e6611f_63_74 replacing mx-n4.

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:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.