Update phpstan/phpstan requirement from ^1.7 to ^1.8

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/1.8.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.7.0...1.8.0)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:development
...

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

composer.json

@@ -42,7 +42,7 @@
     "php-http/psr7-integration-tests": "dev-master",
     "phpspec/prophecy": "^1.15",
     "phpspec/prophecy-phpunit": "^2.0",
-    "phpstan/phpstan": "^1.7",
+    "phpstan/phpstan": "^1.8",
     "phpunit/phpunit": "^9.5",
     "squizlabs/php_codesniffer": "^3.7"
   },