feat: alert banner styling updates

[LeahMarieBush]

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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 🚀

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• Add Asana and Preview links above.
• Conduct thorough self-review.
• Add or update tests as appropriate.
• 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!).
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• 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]

🦋 Changeset detected

Latest commit: 3b73402

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

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Name	Status	Preview	Comments	Updated (UTC)
react-components	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 18, 2024 4:18pm

[LeahMarieBush]

Alert banner styling will remain the same for now