Refactor GlBanner to not use GlCard

gitlab-org/gitlab-ui#2155 (closed) determined than GlBanner should not use a GlCard

the banner design should remain independent from a card, especially considering mode changes (like dark mode).

gitlab-org/gitlab-ui#2324 (closed) also mentions:

GlBanner was rewritten to use GlCard, but conceptually it's not a card, so it should have independent styling/markup.

See gitlab-org/gitlab-ui!3698 (comment 1582375665) for a bit more context.

Definition of done

• GlBanner Vue component is independent to GlCard
• Banner ViewComponent is independent to Card