Closes #8931

Summary

Primary change

Update EuiPopover defaults for modern popover appearance

• hasArrow: false
• offset: 4
• anchorPosition: downLeft

Secondary changes

• Set EuiInputPopover default offset to 2px to retain tighter association for inputs (effectively no visual change)
• Change EuiTourStep to keep the arrow when beacon is present (effectively no visual change)
• Change EuiTourStep story to use popover default instead of hardcoded value
• Update positioning tests to reflect new default behavior

Why are we making this change?

Part of a larger effort to modernize our product UIs.

Screenshots

EuiPopover

(default 4px offset)

EuiInputPopover (visually unchanged)

(default 2px offset)

EuiContextMenu (in Popover; this is an existing storybook)

EuiTour (visually unchanged)

Impact to users

Minimal - visual only

QA

Remove or strikethrough items that do not apply to your PR.

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
  • 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)