fix(rds): unable to create cluster with per-instance parameters (under feature flag) (#27040)

This PR fixes that DatabaseCluster cannot create multiple readers with each parameters.

Closes #27035.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: go-to-k

packages/@aws-cdk-testing/framework-integ/test/aws-rds/test/integ.cluster.js.snapshot/aws-cdk-rds-integ-with-feature-flag.assets.json

@@ -0,0 +1,19 @@
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