[mediaqueries-5] duplication of forced-colors: active and prefers-contrast: forced

Breaking this discussion out of the large mega thread in #2943.

I believe these statements are true.

1. AFAICT, forced-colors: active and prefers-contrast: forced are exact duplications of each other.
2. I am only aware of one implementation of forced colors: on Windows.
3. forced-colors is already implemented in Edge (and possibly in the Chrome HC extension.)

Opinions:

• I'm not sure why two properties are needed. I think I'd heard both that the forced value is intended as a replacement for the forced-colors media feature, and I've also heard that these are intended to be complementary and co-exist. Which is it, and why?
• Since there is only one implementation (Windows), I worry that shoehorning a single forced value into prefers-constrast is unnecessarily limiting. Another platform (iOS, Mac, ChromeOS, Android, Linux) may support a different forced mode in the future, and forced-colors would be easier to extend.

Editorial: prefers-contrast: forced cross-link to forced-colors: active an confirms the duplication. If both are kept, the reciprocal link and explanatory prose should be added, too.