Bump phpunit version to ^9.6

Update the dev dependency constraint for phpunit from ^9.0 to ^9.6 in composer.json

Author: namithj

composer.json

@@ -21,7 +21,7 @@
     "dealerdirect/phpcodesniffer-composer-installer": "^1.0",
     "phpstan/phpstan": "^1.12",
     "phpcompatibility/php-compatibility": "^9.3",
-    "phpunit/phpunit": "^9.0",
+    "phpunit/phpunit": "^9.6",
     "squizlabs/php_codesniffer": "^3.7",
     "roave/security-advisories": "dev-latest"
   },