In SERVER-67393 we made DDL coordinators to implicitly translate a timeseries namespace to its underlying bucket namespace if needed.
This means that DDL coordinator started in binary 6.1 will use the view namespace as main namespace in the coordinator document and additionally they will record also the bucket namespace under an optional filed in their coordinator document.

DDL coordinators in 6.0 were always assuming that the main namespace in the coordinator document will point to the bucket namespace in case the target collection is a timeseries collection.

In order to avoid DDL coordiantors in 6.0 to use a coordinator document created in 6.1 we need to drain all the DDL coordiantors when downgrading to 6.0 FCV.