Improve designs for <Banner> shared component

From #32575

Feedback was:

>Dismiss buttons should be set to primary. This means that on Light theme, it will appear with a black bg. On Dark, it will appear with a whitish bg.

>Could we also check that for banners using the blue bg (info), the top separator colour should be set to border/info/subtle

>We will never use a green gradient in a banner (please check compound components) We also do not have an appropriate token for green gradients. (this is to remove if used anywhere) If you are currently using this, could you let me know where and for what so if needed, I can refer you to the right component and colour please.

>try aim for 32px min between text and start of CTA