REF: make copy keyword in recode_for_categories keyword only
Follows up to https://github.com/pandas-dev/pandas/pull/62000

`recode_for_categories` had a default `copy=True` to copy the passed codes if the codes didn't need re-coding. This PR makes this argument keyword only to make it explicit if the caller wants to copy - to avoid unnecessary copying when blindly using `recode_for_categories`
Thanks @mroeschke 