update http2 config due to deprecation

the previous way to let `http2` follow a `listen` was depracated, it
moved to `http2 on;`

examples/reverse-proxies/nginx/matrix.conf

@@ -1,6 +1,7 @@
 server {
-    listen 443 ssl http2;
-    listen [::]:443 ssl http2;
+    listen 443 ssl;
+    listen [::]:443 ssl;
+    http2 on;
 
     # TODO: add/remove services and their subdomains if you use/don't use them
     # this example is using hosting something on the base domain and an element web client, so example.com and element.example.com are listed in addition to matrix.example.com
@@ -37,8 +38,9 @@ server {
 # settings for matrix federation
 server {
     # For the federation port
-    listen 8448 ssl http2 default_server;
-    listen [::]:8448 ssl http2 default_server;
+    listen 8448 ssl default_server;
+    listen [::]:8448 ssl default_server;
+    http2 on;
 
     server_name matrix.example.com;
 
@@ -93,4 +95,4 @@ server {
     server_name element.example.com;
     listen 80;
     return 404; # managed by Certbot
-}
+}