fix: allow IPv4 and IPv6 to bind to the same port in dual-stack mode

[nflaig]

We are using the same port for IPv4 and IPv6 since ChainSafe/lodestar#8190 but binding to the same port in dual-stack mode doesn't actually work and discv5 just gets stuck trying to open sockets.

this promise never resolves

discv5/packages/discv5/src/transport/udp.ts

Line 84 in 293ff65

const [socket4, socket6] = await Promise.all([

and results in timeout of network worker on startup

consensus-1  | Sep-09 14:43:28.159[network]          info: libp2p worker started peer=<redacted>
consensus-1  | 2025-09-09T14:43:30.209Z discv5:service Starting discv5 service with node id <redacted>
consensus-1  | 2025-09-09T14:43:30.209Z discv5:sessionService Starting session service with node id <redacted>
consensus-1  |  ✖ Error: Timeout: Did not receive an init message from worker after 300000ms. Make sure the worker calls expose().
consensus-1  |     at Timeout._onTimeout (/usr/app/node_modules/@chainsafe/threads/dist/master/spawn.js:35:53)
consensus-1  |     at listOnTimeout (node:internal/timers:588:17)
consensus-1  |     at processTimers (node:internal/timers:523:7)

Setting ipv6Only to true restricts IPv6 sockets to only handle IPv6 traffic, allowing separate IPv4 and IPv6 sockets to bind to the same port number without conflict.