build(deps): update monolog/monolog requirement in /app

Updates the requirements on [monolog/monolog](https://github.com/Seldaek/monolog) to permit the latest version.
- [Release notes](https://github.com/Seldaek/monolog/releases)
- [Changelog](https://github.com/Seldaek/monolog/blob/main/CHANGELOG.md)
- [Commits](https://github.com/Seldaek/monolog/compare/1.24.0...2.9.1)

---
updated-dependencies:
- dependency-name: monolog/monolog
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

app/composer.json

@@ -11,7 +11,7 @@
         "ext-pcre": "*",
         "kamisama/cake-resque": "4.1.2",
         "pear/crypt_gpg": "1.6.7",
-        "monolog/monolog": "1.24.0"
+        "monolog/monolog": "2.9.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^8",