Replace gpt-3.5-turbo with gpt-4o-mini

[julian-risch]

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As of July 2024, gpt-4o-mini should be used in place of gpt-3.5-turbo: https://platform.openai.com/docs/models/gpt-3-5-turbo I found it in a couple of tutorials, for example https://github.com/deepset-ai/haystack-tutorials/blob/13a85a851adde687b713fa085ecf9db18318fc79/tutorials/36_Building_Fallbacks_with_Conditional_Routing.ipynb#L208 The text description of the code cell also needs to be updated then. Here is a list of affected tutorials: https://github.com/search?q=repo%3Adeepset-ai%2Fhaystack-tutorials%20gpt-3.5-turbo&type=code