**How?**
This regression was caused by introducing font-size-0 utility
to a parent element. This was to fix a [spacing issue][1] between
buttons.

This MR decides to use flexbox to fix the spacing issue.

It also intrdocues a `gl-ml-children-sm-3` utility class
that automatically adds the margin between children element.

[1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24350