Remove all IE JS fallbacks and workarounds

[cee-chen]

Summary

The JS version of #6154 and #6161.

Components to QA

• EuiFieldSearch
  • Confirm that typing and then using the clear button works as expected
• EuiTabbedContent
  • Confirm that tabbing or clicking away from the tabs works as expected
  • Confirm that tabbing between tabs works as expected
• EuiComboBox
  • Open Firefox
  • Confirm that tabbing away dismisses the dropdown
  • Confirm that clicking outside the dropdown or input dismisses the combobox dropdown
  • Confirm that clicking options inside the dropdown does not dismiss the dropdown
  • Confirm that clicking anywhere inside the input does not dismiss the dropdown
• EuiIcon
  • Open Edge
  • Open devtools dom inspector and add tabindex="-1" id="test" to any icon
  • In the console, type document.getElementById('test').focus(), press enter
  • In the console, type document.activeElement, press enter and confirm it's the SVG/icon above
  • Click around the icon and tab/shift+tab around it, and confirm you cannot focus the icon via tabbing
  • Edit the tabindex attribute to 0 and then confirm you can now focus the icon via tabbing

Checklist

• Checked in Chrome, Safari, Edge, and Firefox
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

- [ ] Checked in both light and dark modes
- [ ] Checked in mobile
- [ ] Props have proper autodocs and playground toggles
- [ ] Added documentation
- [ ] Checked Code Sandbox works for any docs examples
- [ ] Added or updated jest and cypress tests
- [ ] Updated the Figma library counterpart

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6162/

[chandlerprall]

Is this a breaking change? We don't support IE11 anymore so further unsupporting it it further shouldn't be a break by itself.

[cee-chen]

I labelled it as one to keep the changelog grouped with the other IE CSS removals (some of which are technically breaking changes because we're removing a CSS util and a Sass mixin). It probably technically isn't but it may not be a bad thing to stress we no longer support IE 😅

[chandlerprall]

LGTM, tested changes locally in various browsers to confirm things still work as expected