Tarantool used to send out raft state on subscribe only when raft was
enabled. This was a safeguard against partially-upgraded clusters, where
some nodes had no clue about Raft messages and couldn't handle them
properly.

Actually, Raft state should be sent out always. For example, promote
will be changed to bump Raft term even when Raft is disabled, and it's
important that everyone in cluster has the same term for the sake of promote
at least.

So, send out Raft state to every subscriber with version >= 2.6.0
(that's when Raft was introduced).
Do the same for Raft broadcasts. They should be sent only to replicas
with version >= 2.6.0

Closes #5438