Further Deprecation of Code in ClickHouse Versions Older than 20.6

Further deprecation of already unused code. This is relevant only for users of ClickHouse versions older than 20.6. A "leader election" mechanism is removed from ReplicatedMergeTree, because multiple leaders are supported since 20.6. If you are upgrading from an older version and some replica with an old version is a leader, then server will fail to start after upgrade. Stop replicas with old version to make new version start. After that it will not be possible to downgrade to version older than 20.6. #32140 (tavplubix).