CLN 29547 Replace old string formatting syntax with f-strings
Addresses #29547 for pandas/io/sas/sas_xport.py. I searched through for instances of the `%` operator and `.format`.

- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I noticed that `_read_sas_doc` doesn't seem to be used anymore. I could delete that as well, or we could save that for a separate PR.
