fix(rds): tokens should not be lowercased (#20287)

Fixes #18802

Pretty much does the suggested fix in the issue.

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Author: Zharktas

Diff for: packages/@aws-cdk/aws-rds/lib/cluster.ts

@@ -438,7 +438,7 @@ abstract class DatabaseClusterNew extends DatabaseClusterBase {
     const clusterParameterGroupConfig = clusterParameterGroup?.bindToCluster({});
     this.engine = props.engine;
 
-    const clusterIdentifier = FeatureFlags.of(this).isEnabled(cxapi.RDS_LOWERCASE_DB_IDENTIFIER)
+    const clusterIdentifier = FeatureFlags.of(this).isEnabled(cxapi.RDS_LOWERCASE_DB_IDENTIFIER) && !Token.isUnresolved(props.clusterIdentifier)
       ? props.clusterIdentifier?.toLowerCase()
       : props.clusterIdentifier;
 

Diff for: packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -715,6 +715,7 @@ abstract class DatabaseInstanceNew extends DatabaseInstanceBase implements IData
     }
 
     const maybeLowercasedInstanceId = FeatureFlags.of(this).isEnabled(cxapi.RDS_LOWERCASE_DB_IDENTIFIER)
+    && !Token.isUnresolved(props.instanceIdentifier)
       ? props.instanceIdentifier?.toLowerCase()
       : props.instanceIdentifier;