Update the Gitlab Duo Chat language model section

What does this MR do?

We have moved Chat features to Claude 3 and as such, we should reflect our docs with the correct language models. Unfortunately, not all Duo Chat tools are moved over completely (i.e gitlab_documentation and the ci_editor_assitant), however; the following change would be more reflective of reality.

Changes

Here all the following changes made:

• Duo Chat Claude 2.1 -> Claude-3
• Test generation -> Claude-3-Sonnet

Before	After

Related issues

Revise Duo Chat docs

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