chore(deps): update dependency phpstan/phpstan to v2.1.21 (#1740)

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

Author: renovate[bot]

composer.json

@@ -23,7 +23,7 @@
     "nyholm/psr7": "^1.5",
     "phpbench/phpbench": "^1.2",
     "phpstan/extension-installer": "^1.1",
-    "phpstan/phpstan": "2.1.20",
+    "phpstan/phpstan": "2.1.21",
     "phpstan/phpstan-phpunit": "2.0.7",
     "phpstan/phpstan-strict-rules": "2.0.6",
     "phpunit/phpunit": "^9.5 || ^10.5.21 || ^11",