Use outline style variant for `gl-button` styling

We will need to remove the text-success and outline-success classes in favor of the outline-success gl-button variant after https://gitlab.com/gitlab-org/gitlab-ce/issues/49380 gets addressed.

This came up during !11281 (merged):

@ClemMakesApps started a discussion:

> Do we know if `gl-button` still needs `.text-success` and `.border-success`?