Reconsider stacked button setting for modal breakpoints

Problem

In !1508 (comment 365802749) it was brought up that the stacked buttons take up a lot of space. Having this as a default setting (https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/components/base/modal/modal.scss#L94) can make the button stacking look accidental (especially when stacked buttons aren’t equal width).

Solution

Explore more refined rules for number of actions and rules for when they wrap in an effort to a) save vertical space, and b) have it look more intentional when buttons wrap.

Todo: add visual of proposed solution

/cc @ealcantara