Modernise text contrast rule formatting #1403

Hey folks,
The following pull request is going into final call. This updates the
color-contrast rule to use the latest rule authoring practices. This final
call ends on August 2nd. If you have any comments, please leave a review:

https://github.com/act-rules/act-rules.github.io/pull/1403

Kind regards,

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R