When adding the first shard with a lastLTS FCV to a sharded cluster whose config server has the lastContinuous FCV, an "upgrading from last LTS to last continuous" FCV upgrade path is used to upgrade the shard.

This specific path currently has no reciprocal downgrade path, so we can not currently support an "upgrading to downgrading" path for it. This ticket is to decide how to handle this (such as implementing a downgrade path, or making the initial addShard command fail with a specific error).