Update simple RAG use-case to support latest LangChain syntax

[jacoblee93]

What does this PR do?

Switches the legacy LangChain LLMChain to use expression language, which modernizes/removes some of the parsing.

Who can review?

Feel free to tag members/contributors who may be interested in your PR.

@MKhalusova

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[MKhalusova]

Did you mean to add a code cell here?

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[MKhalusova]

Thanks for improving the example! It looks like there's a code cell missing at the end though. Can you check?

[jacoblee93]

Oh, shoot - yeah I dropped an extra section and forgot to remove the lead-in. Will fix.

[jacoblee93]

Will manually cherry pick the change in the interim.

[MKhalusova]

For the conflict, feel free to override with your changes. Notebook conflicts are a pain to resolve, and the conflicting change is super minor - I can redo it in a separate PR.

[jacoblee93]

Yeah the diff was odd - I readded _Authored by: [Maria Khalusova](https://github.com/MKhalusova)_ which I think was the only thing?

[jacoblee93]

LMK if that looks better

[MKhalusova]

Yeah the diff was odd - I readded _Authored by: [Maria Khalusova](https://github.com/MKhalusova)_ which I think was the only thing?

Yes, that was the only thing added. Thanks for re-adding it!

[MKhalusova]

LGTM, thanks again!

[jacoblee93]

Thank you!