added starttls required for s2s

EXTRAS/example-configurations/conf.d/30-tls.yml

@@ -12,6 +12,7 @@ c2s_ciphers: 'TLS_CIPHERS'
 s2s_ciphers: 'TLS_CIPHERS'
 c2s_protocol_options: 'TLS_OPTIONS'
 s2s_protocol_options: 'TLS_OPTIONS'
+s2s_use_starttls: required
 
 certfiles:
   - /etc/ssl/ejabberd/fullchain.pem