Use a broader PHPUnit constraint

Author: schlessera

composer.json

@@ -15,7 +15,7 @@
         "php-parallel-lint/php-console-highlighter": "^0.5",
         "php-parallel-lint/php-parallel-lint": "^1",
         "phpcompatibility/php-compatibility": "^9",
-        "phpunit/phpunit": ">=4.8 <7",
+        "phpunit/phpunit": "^5 | ^6 |^7 | ^8 | ^9",
         "wp-cli/config-command": "^1 || ^2",
         "wp-cli/core-command": "^1 || ^2",
         "wp-cli/eval-command": "^1 || ^2",