Reverted old version of phpunit for testing on php 7.2

Author: Andrey Helldar

composer.json

@@ -21,7 +21,7 @@
         "andrey-helldar/support": "^2.0|^3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.0"
+        "phpunit/phpunit": "^8.0|^9.0"
     },
     "suggest": {
         "symfony/thanks": "Give thanks (in the form of a GitHub) to your fellow PHP package maintainers"