Previous semantics were to increment unfinishedMigrationFromPreviousPrimary after the async work to drive MigrationCoordinators from previous terms is scheduled, not when it completes. However, after SERVER-127230, the MoveRangeCoordinator only increments it after completion.

unfinished_migration_server_status.js expects that the metric is incremented by the time the MigrationCoordinator document is removed, so we should update the MoveRangeCoordinator to match the previous semantics.