roles/nginx: Use default_server instead of default

Seems to be the new keyword for quite some time now, despite not
causing an error:

    http://nginx.org/en/docs/http/server_names.html

roles/nginx/templates/blank-vhost.conf.j2

@@ -3,14 +3,14 @@
 # clients asking for "example.com" should only get a response if we have
 # a vhost serving that domain.
 server {
-    listen 80 default;
-    listen [::]:80 default;
+    listen 80 default_server;
+    listen [::]:80 default_server;
     server_name _;
 
     return 444;
 }
 server {
-    listen 443 ssl http2 default;
+    listen 443 ssl http2 default_server;
     server_name _;
 
     # "snakeoil" certificate (self signed!)