Update slim/slim requirement from ^4.11 to ^4.12

Updates the requirements on [slim/slim](https://github.com/slimphp/Slim) to permit the latest version.
- [Release notes](https://github.com/slimphp/Slim/releases)
- [Changelog](https://github.com/slimphp/Slim/blob/4.x/CHANGELOG.md)
- [Commits](slimphp/Slim@4.11.0...4.12.0)

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

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

composer.json

@@ -27,7 +27,7 @@
         "monolog/monolog": "^2.9",
         "php-di/php-di": "^6.4",
         "slim/psr7": "^1.6",
-        "slim/slim": "^4.11"
+        "slim/slim": "^4.12"
     },
     "require-dev": {
         "jangregor/phpstan-prophecy": "^1.0.0",