[EuiComboBox] Fixed disabled pills and text contrast

[elizabetdev]

Summary

This PR fixes the EuiComboBox disabled pills and text color contrast.

This issue was originated by:

eui/src/global_styling/mixins/_form.scss

Line 201 in fb286cb

-webkit-text-fill-color: $euiFormControlDisabledColor; // Required for Safari

The text and pills were inheriting this -webkit-text-fill-color in webkit browsers. And in no webkit browsers, there were no disabled styles for the pills and texts. They were getting their default colors.

So I added some styles and also added more DisplayToggles with isDisabled states in a few EuiComboBox examples.

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 and playground toggles
• 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_4901/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4901/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4901/

[elizabetdev]

Thanks @cchaos.

I fixed the remaining issues based on your review. The PR is ready for another pass.

[cchaos]

☹️ Looks like there may be a regression now with the placeholder text when disabled. The text color for disabled is not overriding the placeholder text.

Before

This PR

[elizabetdev]

The placeholder text color is now fixed 🎉

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4901/

[cchaos]

👍 LGTM! Checked again in Safari and FF.