contrib/sni-router: fix bogus 2.4 version reference

The doc said 'Requires mtg ≥ 2.4', but no such release exists — the
latest tag is v2.2.8 (2026-04-07) and #480 is in master since
2026-05-05 awaiting the next release. Replace the speculative version
with an honest pointer to #480 and a 'build from master' note that
stays correct regardless of when the next tag lands.

Refs #511.

contrib/sni-router/README.md

 Caddy's `:8443` listener wrapper so the real client IP still
 propagates to Caddy's logs.
 
-Requires mtg ≥ 2.4 — hostname acceptance for the fronting target was
-added in #480.
+Requires the hostname-acceptance change from #480 (merged 2026-05-05).
+No tagged release contains it yet — build from master until the next
+mtg release ships.
 
 ## ACME (Let's Encrypt) notes