refactor(Question and Answer Chain Node): Use new LangChain's syntax

[burivuhster]

Summary

RetrievalQAChain.fromLLM is deprecated and will be removed in LangChain 1.0.0.
This PR updates the QA Chain node to use the newer syntax recommended by LangChain

The retrieval chain created with createRetrievalChain expects users query in {input} template parameter, while previously RetrievalQAChain expected {question} parameter.

Also, updated version does not wrap the result in an object { text: '...' } and returns a plain text.

As those changes are not backwards compatible, the new version of the node introduced (1.5).

Related Linear tickets, Github issues, and Community forum posts

• https://linear.app/n8n/issue/AI-377/refactor-qa-chain-to-new-syntax

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...s/chains/ChainRetrievalQA/ChainRetrievalQa.node.ts	0.00%	27 Missing ⚠️

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