Update dependency phpstan/phpstan to v2.1.21 (#455)

| datasource | package         | from   | to     |
| ---------- | --------------- | ------ | ------ |
| packagist  | phpstan/phpstan | 2.1.20 | 2.1.21 |

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

composer.json

@@ -36,7 +36,7 @@
         "sort-packages": true
     },
     "require-dev": {
-        "phpstan/phpstan": "2.1.20",
+        "phpstan/phpstan": "2.1.21",
         "phpstan/phpstan-strict-rules": "2.0.6"
     }
 }