Fix zindex on react-datepicker popover

[snide]

Summary

Small one-liner. Because EuiDatePicker doesn't actually use EuiPopover (which uses portals), but instead mimics their look, they need to carry a z-index to them in case they compete with other Dom sibling level components that might have a z-index (like EuiDataGrid, which uses it for focus).

This fix simply applies the z-index we use for these sorts of popovers to the similar element that react-datepicker creates. In most cases, as long as there isn't deep nesting, this should help it stay within the proper z-level versus its peers.

This style of fix was confirmed downstream by @pickypg. I checked the various usages (like inline) to make sure this z-index would not compete with other elements (like Flyouts).

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• [ ] Props have proper autodocs
• Added documentation examples
• Added or updated **[jest tests](https://github.com/elastic/eui/blob/master/wiki/testing.md)**
• 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]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3376/

[cchaos]

👍 I think this makes sense as a temporary solution. I really think we should end up using our own input and popover service to serve the inline version. It would probably also fix #3367 .

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3376/