Update phpstan/phpstan-phpunit requirement from 0.12.18 to 0.12.20

Updates the requirements on [phpstan/phpstan-phpunit](https://github.com/phpstan/phpstan-phpunit) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan-phpunit/releases)
- [Commits](phpstan/phpstan-phpunit@0.12.18...0.12.20)

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

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

Author: dependabot[bot]

composer.json

@@ -22,7 +22,7 @@
         "infection/infection": "^0.20.2",
         "phpstan/extension-installer": "^1.1",
         "phpstan/phpstan": "0.12.90",
-        "phpstan/phpstan-phpunit": "0.12.18",
+        "phpstan/phpstan-phpunit": "0.12.20",
         "phpstan/phpstan-strict-rules": "0.12.9",
         "phpunit/phpunit": "^9.2",
         "psalm/plugin-phpunit": "^0.15.0",