Update Postgres AWS example with actual existing engine version

Avoid

```
'create failed: async create failed: failed to create the resource:
        [{0 creating RDS DB Instance (terraform-20250211145453727900000035): operation
        error RDS: CreateDBInstance, https response error StatusCode: 400, RequestID:
        bbd3565a-5377-445c-8cf9-6268baa6e970, api error InvalidParameterCombination:
        Cannot find version 15.3 for postgres  []}]'
```

Signed-off-by: Yury Tsarev <yury@upbound.io>

Author: ytsarev

examples/postgres-aws-claim.yaml

@@ -10,7 +10,7 @@ spec:
   parameters:
     region: us-west-2
     engine: postgres
-    engineVersion: "15.3"
+    engineVersion: "16.3"
     storageGB: 5
     passwordSecretRef:
       namespace: default