replication: fix updating is_candidate in raft

Currently on applier death `is_candidate` is updated after trying
to start election. So, raft assumes it has healthy quorum and
bumps term even when there's not enough healthy nodes to do that.

Trigger on updating above-mentioned flag is run in
`replicaset_on_health_change`. So, let's move it before executing
`raft_notify_is_leader_seen`, which tries to start election.

Closes #8433

NO_DOC=bugfix