[css-color-6] `contrast-color()` doesn't include the existing syntax from `css-color-5`

[romainmenke]

https://drafts.csswg.org/css-color-6/#colorcontrast

<contrast-color()> = contrast-color(
  [ [ <color> && [ tbd-fg | tbd-bg ] && <target-contrast>? ] |
    [ <color> && [ tbd-fg | tbd-bg ] && <target-contrast>, <color># ] ] )
<target-contrast> = <wcag2>

https://drafts.csswg.org/css-color-5/#funcdef-contrast-color

contrast-color() = contrast-color( <color> max? )

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In css-color-5 there is already a minimal version of this function.
The definition in css-color-6 should also include the max keyword.