chore(deps): update @netlify/plugin-nextjs to 5.15.5

[minimalsm]

Summary

• Updates @netlify/plugin-nextjs from 5.12.0 to 5.15.5

This addresses the build output recommendation:

Outdated plugins
   - @netlify/plugin-nextjs@5.12.0: latest version is 5.15.5
     To upgrade this plugin, please update its version in "package.json"

Test plan

• Verify Netlify deploy preview builds successfully
• No regressions in site functionality

[netlify]

✅ Deploy Preview for ethereumorg ready!

Name	Link
🔨 Latest commit	df0ae76
🔍 Latest deploy log	https://app.netlify.com/projects/ethereumorg/deploys/69730133eaba0000081f58c5
😎 Deploy Preview	https://deploy-preview-17149.ethereum.it
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	7 paths audited Performance: 60 (🟢 up 7 from production) Accessibility: 94 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: 59 (no change from production) View the detailed breakdown and full score reports

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