Use the shells background feature to put it into background.

Puma version 5 had removed the `-d` option.
This should fix #299570.

bin/web_puma

@@ -25,7 +25,7 @@ get_puma_pid()
 
 start()
 {
-  spawn_puma -d
+  spawn_puma &
 }
 
 start_foreground()