Rewritten by @tkajtoch

Summary

React 16 was released in 2017 and reached EOL in 2020. To be able to use modern React features, we can't continue to support it for much longer. We need to deprecate React 16 support in @elastic/eui and supporting dependencies soon.

Acceptance criteria

• EUI and supporting dependencies (e.g., @elastic/eui-theme-borealis) should drop support for React 16
• CI should be updated to not test against React 16 environments
• EUI documentation and our repo's README should be updated to reflect React version compatibility
  • We should include a paragraph about the latest EUI version supporting React 16, and (rough) long-term React support deprecation plans