Draft: Remove utility class provided by @gitlab/ui

What does this MR do and why?

Remove utility class provided by @gitlab/ui

The gl-gap-6 utility was added to @gitlab/ui in gitlab-ui!2889 (merged). This removes it here.

Draft

This requires a version bump to @gitlab/ui which includes gitlab-ui!2889 (merged).

Screenshots or screen recordings

TODO: check places where gl-gap-6 is used (due to cascade order change).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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