Revert "Bump PHPUnit"

This reverts commit 5fde1337d629bd116602f2e67ead75988ae7568f.

composer.json

@@ -18,7 +18,7 @@
         "fzaninotto/faker": "^1.4",
         "mockery/mockery": "^1.0",
         "nunomaduro/collision": "^3.0",
-        "phpunit/phpunit": "^8.3"
+        "phpunit/phpunit": "^8.0"
     },
     "config": {
         "optimize-autoloader": true,