fix(rds): prevent rendering deprecated credentials when creating a database cluster from a snapshot (under feature flag)

[colifran]

This PR fixes a bug where an extra database secret is being generated when an RDS database cluster is being created from a snapshot.

On the DatabaseClusterFromSnapshotProps interface, we deprecated the credentials property and, at the same, introduced snapshotCredentials as the recommended replacement. However, the default behavior associated with the credentials property was not removed as doing so would introduce a breaking change for some users as detailed in this PR. As a result, users just using the recommended snapshotCredentials property to create a new RDS database cluster are seeing an extra, unwanted secret being created.

Closes #23815

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[aws-cdk-automation]

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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).