Update phpstan/phpstan requirement from ^0.12.37 to ^0.12.40

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.37...0.12.40)

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

composer.json

@@ -41,7 +41,7 @@
     "laminas/laminas-diactoros": "^2.3",
     "nyholm/psr7": "^1.3",
     "php-http/psr7-integration-tests": "dev-master",
-    "phpstan/phpstan": "^0.12.37",
+    "phpstan/phpstan": "^0.12.40",
     "phpunit/phpunit": "^8.5",
     "squizlabs/php_codesniffer": "^3.5"
   },