Convert styled-component usage to Emotion vanilla CSS

[cee-chen]

Summary

Follow up to 9125e17 - move away from using euiStyled (which throws errors if plugin(s) using the component aren't wrapped in KibanaStyledComponentsThemeProvider.

See EUI's Emotion FAQ for more information: elastic/eui#6828

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• [ ] Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

[cee-chen]

/ci

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 03aec35

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
triggersActionsUi	761	764	+3

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
triggersActionsUi	1.7MB	1.7MB	+6.5KB

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[ersin-erdal]

LGTM