Bump langchain from 0.1.4 to 0.1.6 in /extensions

Bumps [langchain](https://github.com/langchain-ai/langchain) from 0.1.4 to 0.1.6.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@v0.1.4...v0.1.6)

---
updated-dependencies:
- dependency-name: langchain
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

extensions/backend/requirements.txt

@@ -18,7 +18,7 @@ azure-ai-contentsafety==1.0.0
 requests==2.31.0
 tiktoken==0.5.2
 azure-storage-queue==12.9.0
-langchain==0.1.4
+langchain==0.1.6
 beautifulsoup4==4.12.3
 fake-useragent==1.4.0
 chardet==5.2.0