Deprecate openai_prompt and adapt openai_chatgpt form to handle largest content

Completion API was deprecated with final last update in July 2023 as stated in open api documentation.
See this related issue on the openai-php repo.
Moreover, due to the models filtering, the openai_prompt has no model to use and, thus, do not work in my case.
The openai_chatgpt should now be sufficient with probably more rows for the prompt in order to manage translations, etc.