Workflow update

Author: PDowney

composer.json

@@ -14,7 +14,7 @@
     "php": ">=7.4"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.0",
+    "phpunit/phpunit": "^7.5",
     "yoast/phpunit-polyfills": "^1.0",
     "wp-coding-standards/wpcs": "^2.3",
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7.1"