Switch to Claude3 models

[moritzfl]

This PR applies changes that make the switch to the new API needed for Claude3 models and removes support for the old models.

If you deem it to be necessary, we could do a conditional switch for older models to continue supporting them.

I had no luck with just doing a drop-in replacement of claude-instant-1.2 and claude-2.1 as the messages API generated far too much and continued the conversation by inserting futher questions from "H" (Human) and "A" (Anthrophic) without any of my doing.