relax TLS settings on port 25

Because basically every MTA out there uses opportunistic TLS _in
the best case_, it's actually counter productive to use such strict
settings.

The alternative to a handshake error is often an unencrypted submission,
which is basically the opposite of what strict ssl_protocols and
ssl_ciphers tries to achieve.

Even big and established providers like Amazon SES are incompatible with the current
settings.

This reverts commit 2ddf46ad2b.

core/nginx/conf/nginx.conf

@@ -218,9 +218,9 @@ mail {
       listen 25;
       listen [::]:25;
       {% if TLS and not TLS_ERROR %}
-      ssl_protocols TLSv1.2 TLSv1.3;
+      ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
-      ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;
+      ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA256:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA;
-      ssl_prefer_server_ciphers off;
+      ssl_prefer_server_ciphers on;
       starttls on;
       {% endif %}
       protocol smtp;