Drop PHPUnit 7

Author: GrahamCampbell

composer.json

@@ -22,7 +22,7 @@
     },
     "require-dev": {
         "graham-campbell/analyzer": "^2.4",
-        "phpunit/phpunit": "^7.5|^8.5|^9.0",
+        "phpunit/phpunit": "^8.5|^9.0",
         "php-http/guzzle6-adapter": "^1.1|^2.0"
     },
     "autoload": {