Update default model to GPT-4.1

Not an issue, but a heads up. Since GitHub swapped to GPT-4.1 as default: https://github.blog/changelog/2025-05-08-openai-gpt-4-1-is-now-generally-available-in-github-copilot-as-the-new-default-model/

I guess it's here that needs to be changed? https://github.com/CopilotC-Nvim/CopilotChat.nvim/blob/75653259442a8eb895abfc70d7064e07aeb7134c/lua/CopilotChat/config.lua#L64