Use `default_type` instead of `add_header Content-Type`

The latter can cause duplicate Content-Type headers. Thanks to strcat for notifying me of the issue.

roles/archweb/templates/maintenance-nginx.d.conf.j2

@@ -92,12 +92,12 @@ server {
     error_page 503 /503.html;
 
     location /.well-known/matrix/server {
-        add_header Content-Type application/json;
+        default_type application/json;
         return 200 '{"m.server": "{{ matrix_domain }}:443"}';
     }
 
     location /.well-known/matrix/client {
-        add_header Content-Type application/json;
+        default_type application/json;
         add_header Access-Control-Allow-Origin *;
         return 200 '{"m.homeserver": {"base_url": "https://{{ matrix_domain }}"}, "m.identity_server": {"base_url": "https://matrix.org"} }';
     }