xai/grok-4.20-multi-agent deprecated

The following models are no longer listed in the Agent API model docs, pricing calculator, or examples: Anthropic: `anthropic/claude-opus-4-8`, `anthropic/claude-opus-4-7`, `anthropic/claude-opus-4-6`, `anthropic/claude-opus-4-5`, `anthropic/claude-sonnet-4-6`, `anthropic/claude-sonnet-4-5`, and `anthropic/claude-haiku-4-5` Google: `google/gemini-3-flash-preview`, `google/gemini-3.1-pro-preview`, `google/gemini-3.1-flash-lite`, `google/gemini-3.5-flash`, and `google/gemini-3.5-flash-lite` xAI: `xai/grok-4.3`, `xai/grok-4.20-reasoning`, `xai/grok-4.20-non-reasoning`, and `xai/grok-4.20-multi-agent` For new integrations, use `anthropic/claude-opus-5-5`, `anthropic/claude-sonnet-5-5`, `anthropic/claude-haiku-5-5`, `google/gemini-4-argon`, `google/gemini-3.8-flash`, or `xai/grok-4.7`.