apache-airflow-providers-smtp: No certificate validation on SMTP STARTTLS connections in SMTP provider

Apache Airflow's SMTP provider SmtpHook called Python's smtplib.SMTP.starttls without an SSL context, so no certificate validation was performed on the TLS upgrade. A man-in-the-middle between the Airflow worker and the SMTP server could present a self-signed certificate, complete the STARTTLS...