Update changelog based on model updates

.changes/next-release/api-change-account-36425.json

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+{
+  "type": "api-change",
+  "category": "``account``",
+  "description": "This release enables customers to manage the primary contact information for their AWS accounts. For more information, see https://docs.aws.amazon.com/accounts/latest/reference/API_Operations.html"
+}

.changes/next-release/api-change-ec2-72836.json

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+{
+  "type": "api-change",
+  "category": "``ec2``",
+  "description": "Added support for EC2 M1 Mac instances. For more information, please visit aws.amazon.com/mac."
+}

.changes/next-release/api-change-iotdeviceadvisor-82540.json

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+{
+  "type": "api-change",
+  "category": "``iotdeviceadvisor``",
+  "description": "Added new service feature (Early access only) - Long Duration Test, where customers can test the IoT device to observe how it behaves when the device is in operation for longer period."
+}

.changes/next-release/api-change-medialive-7098.json

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+{
+  "type": "api-change",
+  "category": "``medialive``",
+  "description": "Link devices now support remote rebooting. Link devices now support maintenance windows. Maintenance windows allow a Link device to install software updates without stopping the MediaLive channel. The channel will experience a brief loss of input from the device while updates are installed."
+}

.changes/next-release/api-change-rds-33364.json

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+{
+  "type": "api-change",
+  "category": "``rds``",
+  "description": "This release adds the \"ModifyActivityStream\" API with support for audit policy state locking and unlocking."
+}

.changes/next-release/api-change-transcribe-52569.json

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+{
+  "type": "api-change",
+  "category": "``transcribe``",
+  "description": "Remove unsupported language codes for StartTranscriptionJob and update VocabularyFileUri for UpdateMedicalVocabulary"
+}