Update brianium/paratest dev dependency to ^6 for PHP 8 support.

composer.project.json

@@ -6,7 +6,7 @@
     "require-dev": {
         "behat/mink": "^1.10",
         "behat/mink-selenium2-driver": "^1.6",
-        "brianium/paratest": "^4",
+        "brianium/paratest": "^6",
         "drupal/coder": "^8.3",
         "friends-of-behat/mink-browserkit-driver": "^1.6",
         "mglaman/phpstan-drupal": "^1.1",