Improve condensed EuiTabs focus states

[miukimiu]

Summary

This PR closes #3273.

The EuiTabs condensed version doesn't have any background and relies solely on text underline for focused tabs.

In order to improve these focus states:

• I'm adding a background with the same color as the regular tabs.
• I'm adding a very small left and right padding to make the background look better. This change led me to decreased the margins between tabs.
• I'm also removing the underline when the condensed tab is selected because it was looking odd as referenced on Condensed EuiTabs need better focus states #3273.

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
• [ ] 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_3299/

[cchaos]

🎉 Thank you for fixing this! I think the extra horizontal space inside the tab works as a solution to the background color. It's not a huge visual difference between the old and new.

You'll just need to rebase with master to fix the changelog location since there's been a release since this PR.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3299/