require alpha 2 as a minimum

otherwise, our cp.php assertion in MigrateSettingsTest will fail.

Author: duncanmcclean

composer.json

@@ -7,7 +7,7 @@
         "mustangostang/spyc": "dev-master#dfd9aadc1f5224065d55b42b712c7e99a50a3f4d"
     },
     "require-dev": {
-        "statamic/cms": "^6.0",
+        "statamic/cms": "^v6.0.0-alpha.2",
         "mockery/mockery": "^1.4.4",
         "orchestra/testbench": "^9.6.1 || ^10.0",
         "phpunit/phpunit": "^10.5.35 || ^11.0"