[Emotion] Convert EuiPortal

[miukimiu]

Summary

This PR moves .euiBody-hasPortalContent styles that used to live in _portal.scss to Emotion EuiGlobalStyles (src/global_styling/reset/global_styles.tsx).

The class name .euiBody-hasPortalContent is currently being used in:

• src/components/combo_box/combo_box_options_list/combo_box_options_list.tsx
• src/components/tool_tip/tool_tip_popover.tsx

Checklist

• [ ] Checked 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 and cypress tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• [ ] Updated the Figma library counterpart
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6075/

[chandlerprall]

An alternative (which would be a breaking change) would be to move the global style to the tooltip & combobox components' render. Constance's investigation in (#6055 (comment)) makes me tend to prefer the approach in this PR to retain the global class name and avoid potential, and unknown, breaks.

[miukimiu]

@chandlerprall this PR is ready for review.

Based on work done in #6055, I'm not completely sure what was the decision regarding the .euiBody-hasPortalContent styles. Happy to close this PR if you want to go on a different path.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6075/

[chandlerprall]

This satisfies the conversion for EuiPortal and makes the combobox/tooltip dependency on the global style more explicit. When those other components are converted we can revisit if the global classname is what we want vs. moving it into those components on-demand.