fix(rds): proxy target group does not depend on database instances wh…

…en using writer property for database cluster (#31354)

### Issue # (if applicable)

Closes #31304 .

### Reason for this change



Proxy Target Group should depend on and wait for Aurora instances to be ready before creating CloudFormation resource. (see the issue).

Now, the dependency is added when using a legacy `instanceProps`, but not added when using a `writer` property. 

https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-rds/lib/proxy.ts#L535-L539

(The cluster has `CfnDBInstance` directly when using the `instanceProps`, but it has `AuroraClusterInstance` with `CfnDBInstance` as `defaultChild` when using the `writer`. So the cluster doesn't have the `CfnDBInstance` directly in the latter case.)

### Description of changes



Added the dependency when using a `writer` property instead of `instanceProps`.

### Description of how you validated changes



Both unit and integ tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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