Update NlqMainStack.yaml to Reflect New 16.1 PostgreSQL Engine

Update `NlqMainStack.yaml` to reflect the new 16.1 PostgreSQL engine in two places.

Update the following code:

```text
DBParameterGroupName: "default.postgres15"
OptionGroupName: "default:postgres-15"
```

to

```text
DBParameterGroupName: "default.postgres16"
OptionGroupName: "default:postgres-16"
```