gh-94172: Remove keyfile, certfile and check_hostname parameters (#94173)

Remove the keyfile, certfile and check_hostname parameters,
deprecated since Python 3.6, in modules: ftplib, http.client,
imaplib, poplib and smtplib. Use the context parameter (ssl_context
in imaplib) instead.

Parameters following the removed parameters become keyword-only
parameters.

ftplib: Remove the FTP_TLS.ssl_version class attribute: use the
context parameter instead.