Bump next to next stable patch release (#14289)

### WHY are these changes introduced?

Upgrades `next` from v15.2.4 to v15.2.6 due to security issues with
v15.2.4 ([docs](https://nextjs.org/blog/CVE-2025-66478#fixed-versions)).

### How to 🎩

Run `polaris.shopify.com` locally and confirm site works as expected.



https://github.com/user-attachments/assets/47c5b3e0-6e6f-4efd-bcf8-0e1f998a9fc0

Author: laurkim