[Meta] React 18 Support

[chandlerprall]

Description rewritten by @tkajtoch.

Currently, EUI doesn't support React 18, and we noticed some of the components are behaving unexpectedly or are entirely broken when running on the latest React runtimes with strict mode enabled. We know this may block our users from upgrading to the latest and greatest, and we decided to prioritize the EUI upgrade process to address this issue.

Goals

• Allow users to integrate EUI components with React 18 applications or migrate their existing applications using EUI to React 18
• Keep supporting React >=17 versions in new releases for the time being
• Upgrade EUI docs application to run on the latest React 18 version
• Maintain the same component APIs and props (internal components structure may change)
• Update our testing frameworks to run tests on React 18 and fix any broken tests
• Provide upgrade instructions if necessary

Acceptance Criteria

• All EUI components work according to specification, and their behavior doesn't change between React 18 and older React versions
• EUI docs app runs on React 18
• All unit and e2e tests are passing
• There are no breaking changes to the components' usage or props (stricter prop definitions are allowed - e.g. around the children prop)
• All dependencies using React are updated to versions supporting React 18

Definition of done

• All EUI components are tested using already built automated test suites as well as manually, and any necessary fixes are implemented
• All linked issues to this meta issue are done and closed
• Changes are merged from a feature branch to main
• EUI supporting React 18 is released

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Child issues

Beta Give feedback

1. [EuiPortal] does not work in React 18 + StrictMode #6179
   bug +1
2. [EuiContextMenu] does not work in React 18 + StrictMode #6180
   bug react 18 ⚠️ needs validation +3
3. [EuiToolTip] does not work in React 18 + StrictMode #6181
   bug react 18 +2
4. Drag and drop - use new fork #6187
   dependencies +1
   
5. [React 18] Fix the cause of EuiPopover-related failing tests #6780
   bug +1
6. [React 18] Fix or rewrite EuiIcon tests #6784
   bug +1
7. [React 18] Fix the cause of EuiI18nConsumer-related failing tests #6783
   bug +1
8. [React 18] Enforce Enzyme to render components in React 18 strict runtime #6782
   bug dependencies +2
9. [React 18] Find and cancel async operations when components unmount #6781
   bug +1
10. Update react-router-dom to latest v5 and cleanup related dependencies usage #6891
    task +1
    

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Related issues

• [Meta] Convert enzyme tests to react testing library #5596

[cee-chen]

Follow-up support items:

• StrictMode: [React 18] Strict Mode issues punch list #7017
• Docs: [React 18] Docs issues follow-up #7016