Add Node.js 26 support

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- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [x] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

SSR

### Is your feature request related to a problem? Please describe:

Node.js 26 was released on 2026-05-05. Currently there is no easy way to use Node.js 26 as a build or SSR compute runtime in Amplify Hosting, which means teams cannot start migrating or testing their applications on the new LTS version.

### Describe how you'd like this feature to work

Please add Node.js 26 as a supported runtime in Amplify Hosting, including:
- Build environment (selectable Node.js version during build)
- SSR/compute runtime (for frameworks like Next.js, Nuxt, etc.)

Node.js 26 highlights:
- V8 engine updated to v14.6
- Stable TypeScript support
- Temporal API enabled by default