Upgrade wp-cli-tests to v5 to fix PHPCS dependency conflict

phpcompatibility/php-compatibility dev-develop moved to squizlabs/php_codesniffer ^4.0.1,
which conflicts with wp-coding-standards/wpcs ^3 requiring ^3.x. Upgrading wp-cli-tests
to ^5.0 resolves this by aligning all PHPCS-related dependencies. The explicit
phpcompatibility/php-compatibility requirement is removed since wp-cli-tests v5 pulls it
in transitively as ^10.0.

Also adds phpstan/extension-installer to allow-plugins, as required by wp-cli-tests v5
which added PHPStan support (matching wp-cli/wp-cli upstream config).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sejas

composer.json

@@ -28,9 +28,10 @@
     "wp-cli/wp-cli": "^2.5"
   },
   "require-dev": {
-    "wp-cli/wp-cli-tests": "^4.3.2",
-    "phpcompatibility/php-compatibility": "dev-develop"
+    "wp-cli/wp-cli-tests": "^5.0"
   },
+  "minimum-stability": "dev",
+  "prefer-stable": true,
   "scripts": {
     "behat": "run-behat-tests",
     "behat-rerun": "rerun-behat-tests",
@@ -48,7 +49,8 @@
   },
   "config": {
     "allow-plugins": {
-      "dealerdirect/phpcodesniffer-composer-installer": true
+      "dealerdirect/phpcodesniffer-composer-installer": true,
+      "phpstan/extension-installer": true
     }
   }
 }