Deprecate langchain destination

[flash1293]

The Langchain destination has been superseded by separate destinations for different vector DBs.

This PR makes them invisible in the selection menu in the UI while keeping existing connections running as before.
It also adjusts the documentation to point users to the right place.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
airbyte-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 14, 2023 0:21am

[github-actions]

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[aaronsteers]

Looks good!

[beyondExp]

Is there a version planned for Redis?