Remove gradients in sidebar

[ek-so]

Summary

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If this is your first PR in the EUI repo, please ensure you've fully read through our contributing to EUI wiki guide.

QA

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General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs (using @default if default values are missing) and playground toggles
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
  • Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
  • If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • If applicable, file an issue to update EUI's Figma library with any corresponding UI changes. (This is an internal repo, if you are external to Elastic, ask a maintainer to submit this request)

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8237/
• Storybook: https://eui.elastic.co/pr_8237/storybook
• New docs: https://eui.elastic.co/pr_8237/new-docs

[elasticmachine]

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• Commit: e634cb0