Change config.yaml for function_minimization to use Gemini by default.

[codrut3]

This matches the Quick Start documentation, which asserts the example uses Gemini by default.

I found this issue while trying to follow the documentation to use Openevolve for the first time. I got a Gemini API key but then I found the example requires OpenAI access. I updated the config to use Gemini instead. I ran the evolution and it converges to a solution in 50 iterations.

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