mathlib3

4b312472 - chore(data/*/gcd): move data/*set/gcd to algebra/gcd_monoid/ (#7800)

chore(data/*/gcd): move data/*set/gcd to algebra/gcd_monoid/ (#7800) See discussion of migrating content from `data/` to `algebra/` at [https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/move.20p-adics](zulip). Co-authored-by: Scott Morrison <scott.morrison@gmail.com>