Update changelog based on model updates

Author: aws-sdk-python-automation

.changes/next-release/api-change-bedrockruntime-28777.json

@@ -0,0 +1,5 @@
+{
+  "type": "api-change",
+  "category": "``bedrock-runtime``",
+  "description": "Add support for system tool and web citation response."
+}