Allow downgrade of phpunit to 8 for PHP 7.2 support

Signed-off-by: Christian Wolf <github@christianwolf.email>

composer.json

@@ -12,7 +12,7 @@
       "ext-libxml": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=9.0",
+        "phpunit/phpunit": ">=8.0",
         "nikic/php-parser": "4.2"
     }
 }