fix: add z-index to alert banner

[LeahMarieBush]

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Description

This PR adds a z index to the alert banner component. This is to prevent an overlay blur on the alert banner from the subnav dropdowns.

PR Checklist 🚀

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

• 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!).
• Conduct reasonable accessibility review (use the WAS as a guide or an axe browser plugin until we establish more formal checks).
• 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.

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Name	Status	Preview	Comments	Updated (UTC)
react-components	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 15, 2024 6:28pm

[changeset-bot]

🦋 Changeset detected

Latest commit: b18321f

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	Patch

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[EnMod]

LGTM