Determine the request header version for a given API key and version.
Flexible APIs (per KIP-482) use request header v2
Non-flexible APIs use request header v1 (with client ID)
Note: Request header v0 (no client ID) is not selected automatically.
It exists for protocol completeness but modern clients always send a
client ID (v1+).
Determine the request header version for a given API key and version.
Note: Request header v0 (no client ID) is not selected automatically. It exists for protocol completeness but modern clients always send a client ID (v1+).