Allow nginx to lookup IPv6 addresses

It creates issues with RSPAMD/HFILTER_HOSTNAME_UNKNOWN on v6 enabled
setups see
https://github.com/Mailu/Mailu/issues/2260#issuecomment-1066797661

core/nginx/conf/nginx.conf

@@ -17,7 +17,7 @@ http {
     keepalive_timeout 65;
     server_tokens off;
     absolute_redirect off;
-    resolver {{ RESOLVER }} ipv6=off valid=30s;
+    resolver {{ RESOLVER }} valid=30s;
 
     {% if REAL_IP_HEADER %}
     real_ip_header {{ REAL_IP_HEADER }};
@@ -254,7 +254,7 @@ mail {
     server_name {{ HOSTNAMES.split(",")[0] }};
     auth_http http://127.0.0.1:8000/auth/email;
     proxy_pass_error_message on;
-    resolver {{ RESOLVER }} ipv6=off valid=30s;
+    resolver {{ RESOLVER }} valid=30s;
     error_log /dev/stderr info;
 
     {% if TLS and not TLS_ERROR %}