smp server: fix handshake compatibility by moving server info to SMP v21

[ed-asriyan]

Problem

private routing server master -> server 7.0.0.6 fails during the transport handshake:

Forwarding server: smp://…@localhost:5223
Error: TRANSPORT {transportErr = TEHandshake {handshakeErr = PARSE}}

Root cause

serverInfoSMPVersion was defined as VersionSMP 20, the same version already released as namesSMPVersion (public namespaces resolver, #1784). This made a v20 handshake ambiguous on the wire:

• Released v20 (namespaces) servers advertise max version 20 but send no serverInfo field.
• Server expects serverInfo that isn't there

The issue appeared since 62e39ae - rewrote the SMPServerHandshake parser from (fmap unLarge <$> smpP) <|> pure Nothing to unLarge <$$> smpP, dropping the <|> pure Nothing fallback that had been masking the collision.

Fix

Give server info its own protocol version instead of overlapped v20:

• serverInfoSMPVersion → VersionSMP 21
• currentClientSMPRelayVersion / currentServerSMPRelayVersion → 21

With serverInfoSMPVersion = 21, ifHasServerInfo is false for a v20 handshake, so the parser never looks for serverInfo