k/replicated_partition: use confirmed term as a source of leader epoch

A leader epoch is used by Kafka clients to determine if a replica is up
to date with the leader and to detect truncation.

The leader epoch differs from Raft term as the term is updated when
leader election starts. Whereas the leader epoch is updated after the
state of the replica is determined. Therefore the leader epoch uses
confirmed term instead of the simple term which is incremented every
time the leader election starts.

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit e746f79)

src/v/kafka/server/replicated_partition.h

@@ -173,9 +173,18 @@ class replicated_partition final : public kafka::partition_proxy::impl {
 
     ss::future<std::optional<model::offset>>
       get_leader_epoch_last_offset(kafka::leader_epoch) const final;
-
+    /**
+     * A leader epoch is used by Kafka clients to determine if a replica is up
+     * to date with the leader and to detect truncation.
+     *
+     * The leader epoch differs from Raft term as the term is updated when
+     * leader election starts. Whereas the leader epoch is updated after the
+     * state of the replica is determined. Therefore the leader epoch uses
+     * confirmed term instead of the simple term which is incremented every time
+     * the leader election starts.
+     */
     kafka::leader_epoch leader_epoch() const final {
-        return leader_epoch_from_term(_partition->term());
+        return leader_epoch_from_term(_partition->raft()->confirmed_term());
     }
 
     ss::future<error_code> validate_fetch_offset(