High contrast: push button should be highlighted

[compulim]

Fixes #4718.

Changelog Entry

Fixed

• Fixes #4718. In high contrast mode, Adaptive Card buttons, when pushed, should highlighted properly, by @compulim, in PR #4746

Description

Update AC push button to be highlighted in high contrast mode.

Design

From MDN <system-color>, the Highlight and HighlightText pair will:

• Highlight: Background of selected items
• HighlightText: Text color of selected items

We also explicitly turned off forced-color-adjust, otherwise, it would look:

Specific Changes

• Add styles for @media (forced-color: active)
• Add tests

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)