refactor: Use consistent fallback for GU_APP

Other places in the repository fallback to "rendering".

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: akash1810

dotcom-rendering/src/server/lib/header-middleware.ts

@@ -11,7 +11,7 @@ const getTargetGroupHeaderValue = () => {
 	return 'dev';
 };
 
-const backendApp = process.env.GU_APP ?? 'dev';
+const backendApp = process.env.GU_APP ?? 'rendering';
 const targetGroup = getTargetGroupHeaderValue();
 
 /**