openai: use gpt-4o-mini by default

[AkihiroSuda]

As of July 2024, gpt-4o-mini should be used in place of gpt-3.5-turbo,
as it is cheaper, more capable, multimodal, and just as fast.

https://platform.openai.com/docs/models/gpt-4-turbo-and-gpt-4

PR Checklist

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[AkihiroSuda]

@tmc Could you take a look?

[AkihiroSuda]

@eliben Could you take a look?

[eliben]

@AkihiroSuda thanks for tagging, but I think this is something @tmc will have to approve

[AkihiroSuda]

@tmc Do you still prefer gpt-3.5-turbo to remain as the default?

OpenAI says "As of July 2024, gpt-4o-mini should be used in place of gpt-3.5-turbo, as it is cheaper, more capable, multimodal, and just as fast."
https://platform.openai.com/docs/models/gpt-4-turbo-and-gpt-4

[adamdecaf]

gpt-3.5-turbo-0301, gpt-3.5-turbo-0613, and gpt-3.5-turbo-16k-0613 will be shutdown on Sept 13th 2024.

I got notice from OpenAI after using the defaults in this library. There's a week from today to update the default to gpt-4o-mini (or something newer)