Update phpstan/phpstan requirement from ^1.8.10 to ^2.1.5

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.1.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.8.10...2.1.5)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:development
...

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

composer.json

@@ -24,7 +24,7 @@
   },
   "require-dev": {
     "phpunit/phpunit": "^10.0",
-    "phpstan/phpstan": "^1.8.10",
+    "phpstan/phpstan": "^2.1.5",
     "squizlabs/php_codesniffer": "3.*",
     "phpmd/phpmd": "^2.11",
     "mikey179/vfsstream": "~1.6.8",