[css-contain-2] content-visibility should not be animatable

[vmpstr]

content-visibility is specified as discretely animatable. contain on the other hand is specified as not animatable. Since content-visiblity values change the type of containment values applied, I think it makes sense for content-visibility to not be animatable as well.

I also don't know any specific use-cases that would be solved by letting content-visibility be animatable.

[chrishtr]

Agreed, let's make it non-animatable