Investigate changes in SERVER-46288: Reconfig in 4.2 style with the current config on FCV downgrade

Description

Description of Linked Ticket

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.

Scope of changes

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