MAINT: Deprecate encoding from stata reader/writer
Deprecate the encoding parameter from all Stata reading and writing
methods and classes.  The encoding depends only on the file format and
cannot be changed by users.

- [X] closes #21244
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
