Update phpstan/phpstan requirement from ^0.12.85 to ^0.12.88

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@0.12.85...0.12.88)

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

composer.json

@@ -62,7 +62,7 @@
         "nyholm/psr7": "^1.4",
         "nyholm/psr7-server": "^1.0.1",
         "phpspec/prophecy": "^1.13",
-        "phpstan/phpstan": "^0.12.85",
+        "phpstan/phpstan": "^0.12.88",
         "phpunit/phpunit": "^8.5.13 || ^9.3.8",
         "slim/http": "^1.2",
         "slim/psr7": "^1.3",