Fix warning: nginx 'listen ... http2' directive is deprecated (#405)

Author: magenbrot

bookstack/rootfs/etc/nginx/templates/direct.gtpl

@@ -2,7 +2,8 @@ server {
     {{ if not .ssl }}
     listen 80 default_server;
     {{ else }}
-    listen 80 default_server ssl http2;
+    listen 80 default_server ssl;
+    http2 on;
     {{ end }}
 
     include /etc/nginx/includes/server_params.conf;