Banner > Should banner use gl-card?

The banner makes use of GlCard in https://gitlab.com/gitlab-org/gitlab-ui/-/blob/main/src/components/base/banner/banner.vue#L92, and this issue is to explore whether it should or not. The banner doesn't make use of a card header, and has styles that aren't associated with a card as default options (blue border and fill). Updates to GlCard in the future could lead to visual regression in banners.

Relates to https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/issues/14462