mailman: Show the mailman3 interface by default

All lists have been migrated to mailman3[1] and mailman3 is what users
should use, so show its interface by default and not the mailman2
interface.

[1] 75ac7d09 ("mailman: Fourth and final batch of mailman3 migrated lists")

roles/mailman/templates/nginx.d.conf.j2

@@ -36,6 +36,10 @@ server {
         return 302 $migrated_uri;
     }
 
+    location = / {
+        return 302 /mailman3/lists/;
+    }
+
     # redirect old urls
     location /mailman/ {
         rewrite ^/mailman/(.*) /$1 permanent;