[FLINK-38847][postgres] Update PostgresConnectionPoolFactory#getPoolId method to use username correctly. (apache#4204)

Author: JNSimba

flink-cdc-connect/flink-cdc-source-connectors/flink-connector-postgres-cdc/src/main/java/org/apache/flink/cdc/connectors/postgres/source/PostgresConnectionPoolFactory.java

@@ -47,7 +47,7 @@ public ConnectionPoolId getPoolId(
         return new ConnectionPoolId(
                 config.getHostname(),
                 config.getPort(),
-                config.getHostname(),
+                config.getUser(),
                 config.getDatabase(),
                 dataSourcePoolFactoryIdentifier);
     }