[EuiMarkdownEditor] Fix styling for Amsterdam

[miukimiu]

Summary

This PR fixes the amsterdam styles for the EuiMarkdownEditor:

• Added border-radius similar to the EuiTextArea
• Removed the default shadow and hover shadow when the textarea is focused
• Removed the focus outline styles for the textarea because we have our default focus state (primary bottom border).

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs
• [ ] Added documentation
• [ ] Checked Code Sandbox works for the any docs examples
• [ ] Added or updated jest tests
• [ ] 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_4289/

[cchaos]

LGTM! I'd honestly even consider removing the box-shadow from the default them on focus too because it's such a large component.

Also, for Amsterdam, do you want to round the corners of the bottom of the preview area too?

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4289/

[miukimiu]

Thanks, @cchaos. 🎉

I removed the box-shadow from the default theme and rounded the corners of the bottom of the preview area.