fix(rds): cluster.instanceIdentifiers does not container writer instance ID (#26929)

Cluster writer instance ID is now returned in instanceIdentifiers with reader instance IDs.

Closes #25947.

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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: kirkwat

packages/@aws-cdk-testing/framework-integ/test/aws-rds/test/integ.cluster-instance-id.js.snapshot/Default.assets.json

@@ -0,0 +1,32 @@
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+}