[EuiHeaderLinks] Add `xxs` gutterSize

[cee-chen]

Summary

Will be used in Kibana for a scalable app menu bar related to One Discover

QA

• https://eui.elastic.co/pr_8079/storybook/?path=/story/layout-euiheader-euiheaderlinks--playground&args=gutterSize:xxs

General checklist

• Browser QA
  • [x] Checked in Chrome, Safari, Edge, and Firefox ~- [ ] Checked in both light and dark modes~ ~- [ ] Checked in mobile~ ~- [ ] Checked for accessibility including keyboard-only and screenreader modes~
• Docs site QA
  • [x] Props have proper autodocs (using @default if default values are missing) and playground toggles ~- [ ] Added documentation~ ~- [ ] Checked Code Sandbox works for any docs examples~
• Code quality checklist
  • [x] Added or updated jest and cypress tests ~- [ ] Updated visual regression tests~
• Release checklist
  • [x] 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_8079/
• Storybook: https://eui.elastic.co/pr_8079/storybook
• New docs: https://eui.elastic.co/pr_8079/new-docs

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 38097df16c25d2194e444aaac91103f85571cb5c

[cee-chen]

Emotion made it super easy, barely an inconvenience! 💅