Update slam/phpstan-extensions requirement from ^3.6.1 to ^4.0.0 (#44)

Updates the requirements on slam/phpstan-extensions to permit the latest version.

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

composer.json

@@ -32,7 +32,7 @@
         "narrowspark/coding-standard": "^3.2.0",
         "narrowspark/testing-helper": "^8.0.2",
         "phpunit/phpunit": "^8.4.2",
-        "slam/phpstan-extensions": "^3.6.1"
+        "slam/phpstan-extensions": "^4.0.0"
     },
     "config": {
         "optimize-autoloader": true,