SERVER-64171 added a new step to the FCV downgrade process from 6.0 to 5.x to enforce the maximum chunk size on each sharded collection of the cluster, implemented within ClusterChunksResizePolicyImpl.

This is a best-effort and potentially long-running procedure, which per-collection progress its persisted on disk by setting an optional field on the related config.collections document and cleared upon completion.

The clear logic is nevertheless flawed, since the clear operation (which is meant to target all the entries matching the find clause) gets translated into an update operation with the multi option set to false.