This PR replaces the imports of the Astra DB vector store with the
newly-released partner package, in compliance with the deprecation
notice now attached to the community "legacy" store.
"_Note: depending on your LangChain setup, you may need to install/upgrade other dependencies needed for this demo_\n",
"_(specifically, recent versions of `datasets`, `langchain-openai` and `pypdf` are required, along with `langchain-community`)._"
"_**Note.** the following are all packages required to run the full demo on this page. Depending on your LangChain setup, some of them may need to be installed:_"