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

[julian-risch]

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 It's the default model of OpenAIChatGenerator and OpenAIGenerator. We should treat that change of the default behavior as a breaking change. gpt-3.5-turbo is also used in the code examples of the AnswerJoiner or BranchJoiner.

Here is a list of more mentions in the repo https://github.com/search?q=repo%3Adeepset-ai%2Fhaystack+gpt-3.5-turbo&type=code

• [x] add a deprecation warning
• [ ] change the defaults models for the Generators
• [ ] change the code examples
• [ ] change the tests

[julian-risch]

I opened a related issue in haystack-tutorials https://github.com/deepset-ai/haystack-tutorials/issues/341

[davidsbatista]

is this tied to a release or can the breaking change warning be generic, i.e.:

"in the upcoming releases gpt-3.5-turbo will be replaced by gpt-4o-mini as de default model"

[davidsbatista]

I've added a generic deprecation message not tied to any specific version