fix: set flattenObjectIds to false when calling toObject() for internal purposes

[vkarpov15]

Fix #14935

Summary

insertMany() currently respects schema-level flattenObjectIds option settings, which means if schema sets flattenObjectIds: true then Mongoose will store ObjectIds as strings.

This does not affect save() because save() was fixed in #13648, but that issue still affects insertMany().

In general, internalToObjectOptions should always set flattenObjectIds to true because we rely on those options when trying to get the MongoDB server representation of an object for queries, insertMany(), etc.

We should also backport this fix to 6.x and 7.x.

Examples