Try removing specific phpunit version so we can test with older PHP versions

Author: brandonpayton

composer.json

@@ -16,8 +16,7 @@
 		"wp-coding-standards/wpcs": "^3.1",
 		"phpcompatibility/phpcompatibility-wp": "*",
 		"php-parallel-lint/php-parallel-lint": "^1.3",
-		"yoast/phpunit-polyfills": "2.0.0",
-		"phpunit/phpunit": "8.5.38"
+		"yoast/phpunit-polyfills": "2.0.0"
 	},
 	"config": {
 		"allow-plugins": {