roles/nginx: disable nginx ssl_prefer_server_ciphers

This is apparently the default and recommended by Mozilla's server-
side SSL configurator also recommends. This lets the client choose
the ciphers best for them (and the ciphers in TLS 1.2 and 1.3 are
not currently known to be dangerous).

roles/nginx/templates/https.j2

@@ -29,7 +29,7 @@
     ssl_protocols {{ nginx_ssl_protocols }};
     ssl_ecdh_curve {{ nginx_ssl_ecdh_curve }};
     ssl_ciphers "{{ tls_cipher_suite }}";
-    ssl_prefer_server_ciphers on;
+    ssl_prefer_server_ciphers off;
 
     {# OSCP stapling only works with real certs #}
     {% if use_letsencrypt == true or item.tls_certificate_path %}