In the tenant migration cloner, we separate the _id index from other index builds. We rely on the createCollection code path to build the _id index with default specs, whereas for other indexes, we will use the specs from the donor.

Because of this, if the user specifies v: 1 for their index builds, it seems possible for there to be a mismatch between the versions. Since there is a semantic difference between v:1 and v:2 indexes, we should use the user's specs when building the _id index.