Migrate ios_books SceneDelegate for UIScene

[Wreos]

Part of flutter/flutter#176957

Migrated SceneDelegate.swift to use UI scene using this guideline https://docs.flutter.dev/release/breaking-changes/uiscenedelegate#migration-guide-for-flutter-apps for ios_books app

While testing this sample locally, I noticed a separate existing issue in IosBooks/ViewController.swift: the Google Books response is force-unwrapped. The unauthenticated request currently returns HTTP 429/quota errors in some environments, so json["items"] as! [[String: Any]] can crash at runtime.

I kept this PR scoped to the UIScene migration only, but I can file or send a separate follow-up to make the sample parse the API response defensively, WDYT?

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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[Wreos]

Changelog link in checklist opens 404, since there is no root CHANGELOG.md. Should i create one?