Update changelog based on model updates

.changes/next-release/api-change-dynamodb-19915.json

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+{
+  "type": "api-change",
+  "category": "``dynamodb``",
+  "description": "Increased DynamoDB transaction limit from 25 to 100."
+}

.changes/next-release/api-change-ec2-18043.json

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+{
+  "type": "api-change",
+  "category": "``ec2``",
+  "description": "This feature allows customers to create tags for vpc-endpoint-connections and vpc-endpoint-service-permissions."
+}

.changes/next-release/api-change-sagemaker-86934.json

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+{
+  "type": "api-change",
+  "category": "``sagemaker``",
+  "description": "Amazon SageMaker Automatic Model Tuning now supports specifying Hyperband strategy for tuning jobs, which uses a multi-fidelity based tuning strategy to stop underperforming hyperparameter configurations early."
+}