refactor: change placeholder color

[GaspardMathon]

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• Placeholder on mobile libraries should be in Content tertiary instead of content inactive

[daniel-dumortier]

@GaspardMathon : If you're talking about placeHolder in Vitamin text fields, I am not sure this issue is relevant. Here is a snippet of the code to put placeholder color which shows we use content tertiary (with a "disabled" opacity if the field is disabled) :

var placeholderColor: UIColor {
        if self == .disabled {
            return VitaminColor.Core.Content.tertiary.disabledColor()
        } else {
            return VitaminColor.Core.Content.tertiary
        }
    }