[EuiResizableContainer] Fix z-index ui bugs

[babs20]

Summary

Fixes #5003

This PR changes:

• .euiResizableButton to use z-index: 1 instead of z-index: $euiZLevel1
• .euiResizableToggleButton to z-index: 2 instead of z-index: $euiZLevel1 + 1

Both of these changes fix the UI bug which caused the button's separating line and the button itself to overlap the header when the page is scrolled.

Here is a before and after:

Before:

After:

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
  - [ ] Props have proper autodocs and playground toggles
  - [ ] Added documentation
• Checked Code Sandbox works for any docs examples
  - [ ] Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elizabetdev]

Thanks @babs20 for opening this PR! 🎉 

Let me get our CI running our tests, build a preview, and then I can start a review: jenkins test this.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5164/

[elizabetdev]

Thanks @babs20! LGTM! 🎉

Tested in Chrome, Safari, Edge, and Firefox.