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

## 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.