[chronik-client] Bump protobufjs to ^7.6.5 for Any expansion DoS fix

Summary: Raise protobufjs from ^7.5.5 (resolved 7.5.8) to ^7.6.5 to clear GHSA-wcpc-wj8m-hjx6 (high) and GHSA-f38q-mgvj-vph7 (moderate).

Test Plan:

• pnpm install --frozen-lockfile
• NODE_PATH=./node_modules pnpm --filter chronik-client test
• pnpm audit --audit-level high (protobufjs advisories gone; high 12→11)

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D20339