composer(deps-dev): bump phpstan/phpstan-phpunit from 0.12.18 to 0.12.20

Bumps [phpstan/phpstan-phpunit](https://github.com/phpstan/phpstan-phpunit) from 0.12.18 to 0.12.20.
- [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
  update-type: version-update:semver-patch
...

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

composer.json

@@ -27,7 +27,7 @@
     "phpstan/extension-installer": "^1.1.0",
     "phpstan/phpstan": "~0.12.84",
     "phpstan/phpstan-deprecation-rules": "~0.12.6",
-    "phpstan/phpstan-phpunit": "~0.12.18",
+    "phpstan/phpstan-phpunit": "~0.12.20",
     "phpstan/phpstan-strict-rules": "~0.12.9",
     "phpunit/phpunit": "^8.5.15",
     "psalm/plugin-phpunit": "~0.15.1",