Update Modal component with new button component

We need to make sure that all components in GitLab UI use the latest button component.

&1049 (comment 283029837) discusses the strategy to get the button component fully integrated after which we can update the components as well.

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Align the production gl-button with the design system, which means merge changes of the gl-new-button into the gl-button. This will allow the gl-modal to pass the visual_gitlab_integration job.

If this is completed the folowing task originally from gitlab-org/gitlab-services/design.gitlab.com#282 (closed) can be completed

• Banner on vue tab that warns of incorrect styling is removed from Pajamas
• Remove banner implemented by !1064 (merged)