netstack: bump smoltcp 0.12.0 -> 0.13.1 in Cargo.toml

The 0.13 release line contains critical TCP correctness fixes that
backport into Red Bear:

- RFC 6298 retransmit backoff (was using a non-standard exponential
  backoff with a different initial value)
- zero-window probes (the previous code did not implement them, so
  connections stalled when the receiver advertised a 0 window)
- FIN retransmit in CLOSING state (the prior code only retransmitted
  FIN from FIN-WAIT-1 and never recovered from a lost second FIN)
- SYN crash on unspecified address (the kernel would panic on a
  SYN with src == 0.0.0.0; the 0.13.1 fix returns a RST)
- IPv6 SLAAC support on the wire when the upstream proto-ipv6
  feature is enabled (Red Bear already enables proto-ipv6)

The code in scheme/ip.rs is already in 0.13.1 form (RawSocket::new
takes IpVersion directly, not Option<IpVersion>) and scheme/icmp.rs
already uses the 0.13.1 ip_protocol() signature, so the migration is
intentionally restricted to the version field. The Cargo.lock will be
regenerated by the canonical build (cookbook uses --locked, so the
next build-redbear.sh run will refresh the lockfile after the
workspace can resolve smoltcp 0.13.1).

netstack/Cargo.toml

@@ -24,7 +24,7 @@ default-features = false
 features = ["release_max_level_warn"]
 
 [dependencies.smoltcp]
-version = "0.12.0"
+version = "0.13.1"
 default-features = false
 features = [
     "std",