Update phpunit min.version

PHPUnit Vulnerable to Unsafe Deserialization in PHPT Code Coverage Handling

Affected Component: PHPT test runner, method cleanupForCoverage()
Affected Versions: <= 8.5.51, <= 9.6.32, <= 10.5.61, <= 11.5.49, <= 12.5.7

Author: bueltge

composer.json

@@ -37,7 +37,7 @@
         "squizlabs/php_codesniffer": "^3",
         "inpsyde/php-coding-standards": "^1",
         "brain/monkey": "^2.2",
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^8.5.52",
         "johnpbloch/wordpress-core": " ^5.0",
         "woocommerce/woocommerce": "^6.6.0"
     },