docs: update GenUI references in Get Started guide

[victoreronmosele]

This PR updates the GenUI references in the Get Started guide.

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[gemini-code-assist]

Code Review

This pull request updates the documentation in 'get-started.md' to align with recent API changes in the 'genui' and 'firebase_ai' packages, including updated dependency names, simplified model and chat message initializations, and revised widget builder signatures. The review feedback correctly points out that the 'highlightLines' parameter on a code block needs to be updated to reflect the shifted lines.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 1b1aadf):

https://flutter-docs-prod--docs-pr13527-fix-update-genui-refer-a1pplqcc.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 1b1aadf):

https://flutter-dev-230821--www-pr13527-fix-update-genui-refer-jc6k8j2l.web.app