Update nginx.conf.gtpl to suppress: 'listen ... http2' directive is d…

…eprecated

Fixes issue #3791
-----
Changed this:
listen 443 ssl http2;

To this:
listen 443 ssl;
http2 on;
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Source: https://www.nginx.com/blog/nginx-plus-r30-released/#Important-Changes-in-Behavior-

nginx_proxy/rootfs/etc/nginx/nginx.conf.gtpl

@@ -34,7 +34,8 @@ http {
     server {
         server_name _;
         listen 80 default_server;
-        listen 443 ssl http2 default_server;
+        listen 443 ssl default_server;
+        http2 on;
         ssl_reject_handshake on;
         return 444;
     }
@@ -60,9 +61,11 @@ http {
         ssl_dhparam /data/dhparams.pem;
         
         {{- if not .options.real_ip_from  }}
-        listen 443 ssl http2;
+        listen 443 ssl;
+        http2 on;
         {{- else }}
-        listen 443 ssl http2 proxy_protocol;
+        listen 443 ssl proxy_protocol;
+        http2 on;
         {{- range .options.real_ip_from }}
         set_real_ip_from {{.}};
         {{- end  }}