Fix ciphers not configured for STARTTLS when postfix < 3.1

[jun66j5]

I tried generated configuration with Postfix 2.10.1, however I noticed the ciphers for STARTTLS are not changed. Investigating, default of smtpd_tls_ciphers is medium on postfix >= 3.1.0, however export on postfix < 3.1.0.

I consider that the configuration should have smtpd_tls_ciphers line.

• Postfix 3.1.0: https://github.com/vdukhovni/postfix/blob/v3.1.0/postfix/src/global/mail_params.h#L1320
• Postfix 3.0.0: https://github.com/vdukhovni/postfix/blob/v3.0.0/postfix/src/global/mail_params.h#L1319