I 100% disagree with the real reason the OP wants to sue. If they had legitimate grounds that this iPad had to run on software prior to iOS 8, I could understand a letter to Cook but not a lawsuit.
Look, I am 100% not happy losing a very important iOS feature (to me) with iOS 10 (being able to take screenshots of iTunes DMR movies, which I knew they were going to axe eventually); unfortunately I lost iOS 9.3.5 with my iPhone hack / activation lock fiasco.
but as everyone else has said, OP, you elected to upgrade your iPad 2 to iOS 8.0. That's the risk you took.
BTW my mom's iPad 2 is running iOS 5 and it now runs like crap because everything else has moved on, while my mom refuses to upgrade. Just sayin'.