* Upgrade confluent-kafka to 2.8.0 now that we found how to avoid SSL validation errors

See https://phabricator.wikimedia.org/T386092

 for the details, but we
found the right config change, allowing us to upgarde `confluent-kafka`
to 2.8.0, with a dependency on `httpx` instead of `requests` and still
be able to validate self-signed certificates.

Signed-off-by: Balthazar Rouberol <brouberol@wikimedia.org>
Bug: T386092