tests: add PHPStan deprecation rules

Author: justlevine

composer.json

@@ -24,7 +24,8 @@
 		"wp-coding-standards/wpcs": "~3.1.0",
 		"phpcompatibility/phpcompatibility-wp": "~2.1.3",
 		"phpstan/phpstan": "^2.1.2",
-		"yoast/phpunit-polyfills": "^1.1.0"
+		"yoast/phpunit-polyfills": "^1.1.0",
+		"phpstan/phpstan-deprecation-rules": "^2.0"
 	},
 	"config": {
 		"allow-plugins": {