Which AWS service will support this requirement with the LEAST amount of operational overhead?

Explanations:

AWS Systems Manager is primarily used for operational management of resources, not specifically for credential management. While it can store some parameters, it does not provide the same specialized functionality for secrets management as other services.

AWS Secrets Manager is specifically designed to securely store, manage, and rotate credentials, making it the best choice for managing database user credentials with minimal operational overhead. It provides built-in functionality for automatic rotation of secrets.

AWS License Manager is focused on managing software licenses, not on managing user credentials or secrets. It does not provide features for credential rotation or storage.

AWS Managed Services provides operational management for AWS resources but does not specialize in credential management. It does not focus on the automated rotation of database credentials.