meta: bump UPSTREAM.md ethp2p pin when github.com/ethp2p/ethp2p main advances

[ch4r10t33r]

Context

UPSTREAM.md pins reference commit db6e9417d0bbab9ded28aa3053211cdecff402ac. As of this check, ethp2p main HEAD equals that commit (no drift).

When ethp2p/main moves forward:

1. Diff proto/*.proto against upstream (broadcast/pb, protocol/pb, broadcast/rs/pb, gossipsub RPC as applicable).
2. Run zig build test and fix golden bytes / constants.
3. Update the pin hash and changelog bullets in UPSTREAM.md (see existing process in that file).

Goal

Track the maintenance task so proto/constant drift is not missed after upstream releases.

Closed #15 covered initial pinning; this issue is for ongoing sync.