nginx: use a compression level of 5 in recommended configuration

While there is little gain of space to use a compression level of 9,
the CPU usage is significant. Many experiments point to use something
between 4 and 6. For example:

 - https://mjanja.ch/2015/03/finding-the-nginx-gzip_comp_level-sweet-spot/
 - https://github.com/h5bp/server-configs-nginx/blob/3bda5b93edba147d51760e900c2079828a7dc274/nginx.conf#L93

nixos/modules/services/web-servers/nginx/default.nix

@@ -92,7 +92,7 @@ let
         gzip on;
         gzip_disable "msie6";
         gzip_proxied any;
-        gzip_comp_level 9;
+        gzip_comp_level 5;
         gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
         gzip_vary on;
       ''}