feat(group_theory/nilpotent): add nilpotency_class inequalities (#11585)
feat(group_theory/nilpotent): add nilpotency_class inequalities (#11585)

Every theorem that proves `nilpotency G'` (e.g. for subgroups, images,
preimages) should be accompanied by a lemma relating their
`nilpotency_class`, so add thse lmmeas for subgroups and preimages.

Also add nilpotency lemmas for surjective homomorphisms and quotions,
including nilpotency_class lemmas.







leanprover-community/mathlib,leanprover-community/mathlib