Update log configuration to match SymfonyCloud recos (#138)

See https://symfony.com/doc/current/cloud/cookbooks/go_live.html#logging

config/packages/prod/monolog.yaml

@@ -8,7 +8,7 @@ monolog:
             buffer_size: 50 # How many messages should be saved? Prevent memory leaks
         nested:
             type: stream
-            path: "%kernel.logs_dir%/%kernel.environment%.log"
+            path: php://stderr
             level: debug
         console:
             type: console