[Whisper] Better error message for outdated generation config
# What does this PR do?

Gives a better error message in the case that a user tries using an outdated generation config with the new generation arguments `language` and `task` (as described in https://github.com/huggingface/transformers/issues/25084#issuecomment-1653722724).

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25298). All of your documentation changes will be reflected on that endpoint.