CLN: frequency.get_offset always return copy
Closes #11192. 

Fix 2 points which should not affect to normal users (thus no release note descriptions).
- get_offset (#11192): It returns cached offset, overwriting it may leads to unexpected results. `pandas` public methods doesn't use them as it is. Thus it should not affect to almost all users, except who explicitly uses `frequencies.get_offset`
- get_offset_name: This looks leagacy impl which raises `ValueError` for `BDay` with `n != 1` which is valid input. I've marked this as deprecated, but it can be removed immediately as non-public.
