build(deps-dev): update phpunit/phpunit requirement

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-9.5.md)
- [Commits](sebastianbergmann/phpunit@9.5.19...9.5.20)

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

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

Author: dependabot[bot]

composer.json

@@ -31,7 +31,7 @@
     "phpstan/extension-installer": "^1.1",
     "phpstan/phpstan": "^0.12",
     "phpstan/phpstan-phpunit": "^0.12",
-    "phpunit/phpunit": "^9.5.19",
+    "phpunit/phpunit": "^9.5.20",
     "slam/phpstan-laminas-framework": "^0.12",
     "squizlabs/php_codesniffer": "^3.6"
   },