Fix incorrect migration notes for `base-0004`

[leonschh]

Problem

base-0004 (Engine API Usage) in app/upgrades/data/changes.ts has incorrect migrationNotes:

Use the registry as the source of accepted TEE signer identities instead of hard-coding Nitro prover signers.

This guidance belongs to base-0001 (Multiproofs), where TEE Provers, Nitro prover signers, and TEEProverRegistry are documented.

For base-0004, the notes should instead reflect the Engine API requirements documented in the engine-api-usage specification, including the V5 payload envelope, engine_newPayloadV4, and empty blob/execution request fields.

Fix

Updated migrationNotes for base-0004 to match its Engine API requirements.
PR: #73