Function api_versions_max_version 

pub fn api_versions_max_version(unstable: UnstableApiVersions) -> i16

Highest ApiVersions version a listener serves under unstable.

It is the same maximum its api_keys table advertises for API 18: Kafka 4.3.1’s v4 by default, and trunk’s KIP-1242 v5 under unstable.api.versions.enable.