Bump phpunit to ^11 for SilverStripe 6 compatibility.

SS6 framework requires sebastian/diff ^6.0, which conflicts with
phpunit ^9's requirement of sebastian/diff ^4.0.6. CI was unable
to resolve dependencies.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: adiwidjaja

composer.json

@@ -37,7 +37,7 @@
         "deeplcom/deepl-php": "^1.12"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9",
+        "phpunit/phpunit": "^11",
         "squizlabs/php_codesniffer": "^3",
         "wernerkrauss/silverstripe-rector": "^1"
     },