Update UnderlinePanels to place provided className on outermost element

[joshblack]

Closes #6253

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Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
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• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	92.01 KB (0%)
packages/react/dist/browser.umd.js	92.29 KB (0%)

[mperrotti]

Since we're moving the passed className to the custom element, this update might have unintentional effects for existing usage: https://primer-query.githubapp.com/?query=%22UnderlinePanels%22+attribute%3AclassName

We should test before merging. If it breaks things, we could include this change in the v38 release.