chore(deps): update dependency phpunit/phpunit to v9.6.21

Author: renovate[bot]

Diff for: composer.json

@@ -18,7 +18,7 @@
   "require-dev": {
     "dms/phpunit-arraysubset-asserts": "0.5.0",
     "friendsofphp/php-cs-fixer": "*",
-    "phpunit/phpunit": "9.6.19",
+    "phpunit/phpunit": "9.6.21",
     "php-coveralls/php-coveralls": "2.7.0",
     "squizlabs/php_codesniffer": "3.10.2"
   },