raft: reply to heartbeats with group_unavailable if addressed to wron…

…g node

Previously, we replied with "failure", which updated the last heartbeat
timestamp and gave the leader an impression that the group is actually there.
Reply with group_unavailable, which is more appropriate here.

(cherry picked from commit 25541ce)

src/v/raft/consensus.cc

@@ -3955,7 +3955,7 @@ reply_result consensus::lightweight_heartbeat(
           target_node,
           _self,
           source_node);
-        return reply_result::failure;
+        return reply_result::group_unavailable;
     }
 
     /**
@@ -4010,7 +4010,7 @@ ss::future<full_heartbeat_reply> consensus::full_heartbeat(
           target_vnode,
           _self,
           source_vnode);
-        reply.result = reply_result::failure;
+        reply.result = reply_result::group_unavailable;
         co_return reply;
     }
     /**