chore(deps): update dependency monolog/monolog to v3

Author: renovate-bot

appengine/standard/symfony-framework/composer.json

@@ -3,7 +3,7 @@
         "php": "^7.1.3"
     },
     "require-dev": {
-        "monolog/monolog": "^2.0",
+        "monolog/monolog": "^3.0",
         "nikic/php-parser": "^4.0",
         "google/cloud-logging": "^1.14"
     }