This only needs to go into v3.6, because v3.8 will remove the addShard FCV & WireVersion check courtesy of changes to be done in SERVER-32519

We bump the wire version (here) if we're fully upgraded, upgrading or downgrading. However, in addShard (here) we only prevent adding the shard if we're fully upgraded.

So, currently, if we're in the process of upgrading, we'll allow an old binary version shard to be added, and then we'll presumable finish upgrading and no server will talk to it ever again.