Use text-bison instead of codechat-bison for ETV

What does this MR do and why?

Use text-bison instead of codechat-bison for explain this vulnerability as this has been found to provide better results from our prompts in our analysis.

How to set up and validate locally

The only visible change should be an improvement in the quality of the ETV responses (and a mild improvement in response time), neither of which can be objectively quantified for validation.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.