Update phpstan/phpstan requirement from ^1.0 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@1.0.0...1.8.2)

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

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

Author: dependabot[bot]

composer.json

@@ -84,7 +84,7 @@
         "php-parallel-lint/php-parallel-lint": "^1.3",
         "phpbench/phpbench": "^1.2",
         "phpstan/extension-installer": "^1.1",
-        "phpstan/phpstan": "^1.0",
+        "phpstan/phpstan": "^1.8",
         "phpstan/phpstan-deprecation-rules": "^1.0",
         "phpstan/phpstan-phpunit": "^1.0",
         "phpstan/phpstan-strict-rules": "^1.0",