Use ChatGenerator instead of Generator

[julian-risch]

We should make the example in cookbook recipes work with ChatPromptBuilder and ChatGenerator instead of Generator as we want to shift to ChatGenerators. Or consider taking outdated recipes offline.

I searched for OpenAIGenerator with this query but there might be other Generators too.

• [ ] apify_haystack_rag_web_browser.ipynb
• [x] newsletter-agent.ipynb
• [ ] openapitool.ipynb
• [ ] using_hyde_for_improved_retrieval.ipynb
• [x] llama3_rag.ipynb
• [x] query-expansion.ipynb
• [ ] query_decomposition.ipynb
• [ ] apify_haystack_rag.ipynb
• [ ] astradb_haystack_integration.ipynb
• [ ] conversational_rag_using_memory.ipynb
• [ ] rag_eval_uptrain.ipynb
• [ ] hackernews-custom-component-rag.ipynb
• [ ] metadata_enrichment.ipynb
• [ ] extracting_metadata_filters_from_a_user_query.ipynb
• [ ] model_explorer_streaming.ipynb
• [x] rag_eval_ragas.ipynb but this one I'd say is expected to use Generator currently
• [ ] apify_haystack_instagram_comments_analysis.ipynb
• [ ] chat_with_SQL_3_ways.ipynb There is a subissue for this one https://github.com/deepset-ai/haystack-cookbook/issues/212