[LLM] Fix langchain UT (#8929)

* Change dependency version for langchain uts

* Downgrade pandas version instead; and update example readme accordingly

.github/workflows/llm_unit_tests.yml

@@ -183,5 +183,6 @@ jobs:
         run: |
           pip install -U langchain==0.0.184
           pip install -U chromadb==0.3.25
+          pip install -U pandas==2.0.3
           pip install -U typing_extensions==4.5.0
           bash python/llm/test/run-llm-langchain-tests.sh

python/llm/example/langchain/README.md

@@ -10,6 +10,7 @@ Follow the instructions in [Install](https://github.com/intel-analytics/BigDL/tr
 ```bash
 pip install langchain==0.0.184
 pip install -U chromadb==0.3.25
+pip install -U pandas==2.0.3
 pip install -U typing_extensions==4.5.0
 ```