Update all README.md example to use Gemini Flash 2.5

[davideast]

All examples in the README.md default to Gemini Flash 2.0, this is outdated and should reference Flash 2.5 or 2.5 Pro.

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.