Partial fix link references color in high contrast

[compulim]

Fixes #4907.

Changelog Entry

Fixed

• Fixes #4907. Link references in high contrast mode should use LinkText color, by @compulim, in PR #4908

Description

Link references of non-URL citations should use LinkText for color.

For "external link" icon in Markdown, they are shown in black and we have a bug #4906 to track the issue.

Specific Changes

• Updated StyleSet/LinkDefinitions.ts for high contrast
• Added "run options" for axe-core/shift-left to disable color-contrast rule for tests in high contrast
  • The placeholder (grayed out) texts are in green and they are not acceptable on color contrast. However, this is OS default

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

Review Checklist

This section is for contributors to review your work.

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