Specify Node.js 20 requirement for langchain install (#1534)

## Overview
Added Node.js version requirement for installation commands.

## Type of change

**Type:** Update existing documentation

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## Checklist
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- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
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- [x] I have updated navigation in `src/docs.json` if needed

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## Additional notes
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Author: mcavdar

src/oss/langchain/install.mdx

@@ -25,18 +25,22 @@ To install the LangChain package:
     <CodeGroup>
     ```bash npm
     npm install langchain @langchain/core
+    # Requires Node.js 20+
     ```
 
     ```bash pnpm
     pnpm add langchain @langchain/core
+    # Requires Node.js 20+
     ```
 
     ```bash yarn
     yarn add langchain @langchain/core
+    # Requires Node.js 20+
     ```
 
     ```bash bun
     bun add langchain @langchain/core
+    # Requires Node.js 20+
     ```
     </CodeGroup>
 :::