Steam? Should I just use my fingernails? It's on the cardboard slip cover, for fucks sake why is this legal
I used to work at an independent video game store years ago and surprising enough, we would use a fresh price sticker to pull sticker residue off. Keep in mind most game packaging is plastic so, your results may vary.
How glossy is the cardboard? Depending on that, the cleaner you use might soak into and darken the cardboard, making it look even worse.
Worst case, a very light brushing of water. Then lightly scrape with your fingernail, and/or rub with your thumb in a circular pattern.