$changeStream pipelines currently use the collation specified on the operation, and error if the collation is non-simple. $changeStream pipelines should instead resolve the collation by using the operation collation if specified, and otherwise inheriting the collection default collation. This will keep their behavior in accordance with other aggregation pipelines, and ensure that there is no behavior change when we start supporting collation on $changeStream pipelines. For now, they should continue to error if the resolved collation is non-simple. Note that resolving the collation requires taking a collection lock (which $changeStream pipelines do not currently do).