refactor(Banner): Update tokens

What does this MR do?

• Update banner design tokens.
• Visually enhances the difference between banners and alerts (dark mode in particular) since they can both be present in the same context.
• Updates the promotional banner to use purple because this is specifically for "a promotional message to encourage exploring or upgrading to a new feature," and purple "is associated with the GitLab brand, it communicates an affinity with the brand, and is used for things like illustrations or company-related announcements in the product." The promo banner is for highlighting a product feature. Purple is also used for the tier badge, which relates to product features.
• The 950 step is used for dark backgrounds, for info feedback in particular this also impacts highlighted table rows and other elements that use this to highlight an element. It's intentional to use this darker color in those places too.

Screenshots or screen recordings

Variant	Before	After
Introduction (light)
Introduction (dark)
Promotion (light)
Promotion (dark)

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.