langchain.agents: corrected documentation (#28986)
**Description:**
This PR updates the codebase to reflect the deprecation of the AgentType
feature. It includes the following changes:
Documentation Update:
Added a deprecation notice to the AgentType class comment.
Provided a reference to the official LangChain migration guide for
transitioning to LangGraph agents.
Reference Link: https://python.langchain.com/docs/how_to/migrate_agent/
**Twitter handle:** @hrrrriiiishhhhh
---------
Co-authored-by: Chester Curme <chester.curme@gmail.com>
