feat(group_theory/nilpotent): add upper_central_series_eq_top_iff_nilpotency_class_le (#11670)
feat(group_theory/nilpotent): add upper_central_series_eq_top_iff_nilpotency_class_le (#11670)

and the analogue for the lower central series.







leanprover-community/mathlib,leanprover-community/mathlib