Migration from vertex_ai.preview to vertex_ai.generative_model

[Amnah199]

Summary and motivation

Current integration of VertexAIGemini uses vertex_ai.preview package which has some experimental features. We should migrate to a more stable vertetx_ai.generative_model package.

Checklist

Tasks

Preview Give feedback

1. The changes are merged in the main branch (Code + Docstrings)
2. New package version declares the breaking change
3. The package has been released on PyPI
4. Docs at https://docs.haystack.deepset.ai/ were updated
5. Integration tile on https://github.com/deepset-ai/haystack-integrations was updated
6. Notebooks on https://github.com/deepset-ai/haystack-cookbook were updated (if needed)
7. Tutorials on https://github.com/deepset-ai/haystack-tutorials were updated (if needed)
8. Articles on https://github.com/deepset-ai/haystack-home/tree/main/content were updated (if needed)

[julian-risch]

Look out for breaks of the serialization