feat: alert banner styling updates

[LeahMarieBush]

🎟️ Asana Task 🔍 Preview Link

---

Description

This PR updates the styling of the alert banner. As part of these changes, there is no longer a close button on the alert so that code has been removed.

PR Checklist 🚀

Items in this checklist may not may not apply to your PR, but please consider each item carefully.

• [x] Add Asana and Preview links above.
• [x] Conduct thorough self-review.
• [x] Add or update tests as appropriate.
• [x] Conduct reasonable cross browser testing for both compatibility and responsive behavior (We have a Sauce Labs account for this, if you don't have access, just ask!).
• [x] Conduct reasonable accessibility review (use the WAS as a guide or an axe browser plugin until we establish more formal checks).
• [x] Identify (in the description above) and document (add Asana tasks on this board) any technical debt that you're aware of, but are not addressing as part of this PR.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: b0af764ca1002658e12966ec85b4181cb148bf12

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------------------- | ----- | | @hashicorp/react-alert-banner | Minor |

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
react-components	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 16, 2024 4:20pm

[LeahMarieBush]

Alert banner styling will remain the same for now

[LeahMarieBush]

Alert banner styling will remain the same for now

After a discussion in Asana, the alert banner changes will be included for the nav updates