Safe reconfig introduces a new field on the config which will make 4.2 binary to fail to start up. Now that we allow 4.4 binary to use and store a config with the new field. We cannot just remove the extra field on downgrade on each node as in
SERVER-45092, since in mixed version replset, a reconfig can propagate the config with the extra field to 4.4 binary with FCV 4.2. That will fail 4.2 binary after downgrade.
Instead, we'll do a reconfig with a higher config version but no term if the last config has a term field with the same configuration and wait for it to replicate to all nodes before downgrading FCV.
Also remove the FCV check in ReplSetHeartbeatArgsV1::addToBSON, we don't need to check FCV to decide whether to include "configTerm" in heartbeats.