Update requirements.txt

Author: dishaprakash

samples/langchain_on_vertexai/requirements.txt

@@ -1,8 +1,8 @@
-google-cloud-aiplatform[reasoningengine,langchain]==1.109.0
+google-cloud-aiplatform[reasoningengine,langchain]==1.90.0
 langchain-google-alloydb-pg==0.12.0
-langchain-google-vertexai==2.0.28
+langchain-google-vertexai==2.0.20
 google-cloud-resource-manager==1.14.2
-langchain-community==0.3.27
+langchain-community==0.3.26
 # Required to fix: "PydanticUndefinedAnnotation: name 'SafetySetting' is not defined"
 # Todo: remove after upstream issue is fixed: https://github.com/langchain-ai/langchain/issues/28271
 pydantic==2.11.7