Make Iron Ore spawn deeper

Playtesting 21w07a on survival, it felt totally off to have such an early access to Iron when the underground is so much deeper and has more room for progression. On average, with 3 different seeds, I got enough ingots for a bucket and some armor in under 10 minutes. I don't want to have a Shield on my first night.

Copper should generate where Iron currently does, while Iron probably needs to generate something like 16 blocks deeper than currently. People would finally use Leather and Stone for a longer time.