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/main/ChangeLog-9.5.md) - [Commits](sebastianbergmann/phpunit@9.5.20...9.5.21) --- 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": "^9.5.20",
+ "phpunit/phpunit": "^9.5.21",
  "slam/phpstan-laminas-framework": "^0.12",
  "squizlabs/php_codesniffer": "^3.7"
  },