chore(group_theory/order_of_element): move some lemmas (#7031)
I moved a few lemmas to `group_theory.subgroup` and to `group_theory.coset` and to `data.finset.basic`. Feel free to suggest different locations if they don't quite fit. This resolves #1563.
Renamed `card_trivial` to `subgroup.card_bot`
Co-authored-by: Julian-Kuelshammer <68201724+Julian-Kuelshammer@users.noreply.github.com>
