fix(openresty): disable server version disclosure in main config template

Add server_tokens off to the default OpenResty main config template, seeded
option template, and agent safe fallback config so responses no longer
expose nginx/OpenResty version numbers in Server headers or error pages.

internal/apps/agent/nginx/manager.go

@@ -198,6 +198,7 @@ events {
 
 http {
     default_type text/plain;
+    server_tokens off;
 
     server {
         listen 80 default_server;