Since 2024-10-02, `gpt-4o` is actually the same as `gpt-4o-2024-08-06`.

We previously used `gpt-4o-2024-08-06`, because it was pointing to a
much better (longer context) model. Since they're both the same now,
we'd better stick to the unpinned model and make it easier for future
users to get upgrades.