Allow earlier PHPUnit

Need this for EOL PHP support.

Author: XedinUnknown

composer.json

@@ -15,7 +15,7 @@
     "psr/simple-cache": "^1.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9",
+    "phpunit/phpunit": "^7.1 | ^8.0 | ^9.0",
     "johnpbloch/wordpress-core": "^5.0",
     "php-stubs/wordpress-stubs": "6.3",
     "brain/monkey": "^2.3",