Function protocol_downgrade_hint 

pub fn protocol_downgrade_hint(
    detected: &ProtocolVersion,
    preferred: &ProtocolVersion,
    cause: &DowngradeCause,
) -> DiagnosticHint

Build the protocol-downgrade hint payload for a proxy + prefer_h3 combination. Public so the BrowserConfig::diagnostic_hints() accessor can call it without re-implementing the message string.