Update ACHostConfig to use accessible TextBlock colors

[corinagum]

Fixes #3812

Changelog Entry

• Fixes #3812. Update adaptiveCardHostConfig to accessible text color-contrasts, by @corinagum in PR #XXX

Description

This updates the host config to use contrast-compliant colors.

I used colors from Fabric UI color scheme to match MSFT colors.

• 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)

[compulim]

Few things:

• Please run npm install. The screenshot generated are from AC 2.5.0, but not the latest AC 2.9.0
• This file is not needed, probably generated because of earlier test failure: __tests__/__image_snapshots__/html/transcript-activity-grouping-js-transcript-with-activity-grouping-test-50-2-snap.png

[compulim]

Looks like there are some legit test failures.

For example, since a new message is added, the scroll bar size changed. The screenshot will need to be updated.

I believe running tests locally before submitting to CI, would catch these test failures sooner.

[corinagum]

@compulim I'm running locally again to check, but this was most likely transient. I mistakenly added a command to the help command on MB that we don't want there, and then immediately removed it. I see it only failed once yesterday, around the time I made that push. The new messages button test only failed once, and the other repeating fails are timeout errors.

I'll report back once I run tests locally again, but I expect them all to pass.