deploy: http2 via the listen directive, for nginx 1.24

The server runs nginx 1.24.0; the standalone `http2 on;` directive only
arrived in 1.25.1, so `nginx -t` rejected the vhost as an unknown directive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/nginx/kadence.timka.su.conf

@@ -29,9 +29,10 @@ server {
 }
 
 server {
-    listen 443 ssl;
-    listen [::]:443 ssl;
-    http2 on;
+    # `listen ... http2` rather than the `http2 on;` directive: this host runs
+    # nginx 1.24, and `http2 on` only arrived in 1.25.1.
+    listen 443 ssl http2;
+    listen [::]:443 ssl http2;
     server_name kadence.timka.su;
 
     ssl_certificate     /etc/letsencrypt/live/kadence.timka.su/fullchain.pem;