CI: Replace deprecated `arduino/setup-protoc` action (Node.js 20 EOL)

[brucearctor]

Summary

The arduino/setup-protoc@v3 action (SHA c65c819552d16ad3c9b72d9dfd5ba5237b9c906b) used in our CI workflows targets Node.js 20, which is deprecated on GitHub Actions.

After September 16, 2026, these actions will fail — not just warn.

Current State

• per-pr.yml: 9 instances of arduino/setup-protoc@c65c819...
• heavy.yml: 1 instance

The arduino/setup-protoc repository has NOT released a Node.js 24 compatible version — the v3 branch's latest commit is still the Node 20 upgrade from Jan 2024.

Proposed Fix

Replace arduino/setup-protoc with a direct protoc binary download from the protobuf releases. This:

1. Eliminates the Node.js runtime dependency entirely
2. Upgrades from protoc 23.x to 27.3
3. Handles all 5 runner platforms (Linux x86_64/ARM64, macOS x86_64/ARM64, Windows)
4. Removes the TODO about arduino/setup-protoc#99

Affected Repos (org-wide)

This same deprecated action is also used in:

• temporalio/sdk-ruby (SHA-pinned, critical)
• temporalio/sdk-typescript (uses ancient @v1)
• temporalio/ui, temporalio/api-go, temporalio/api (various @v3 references)

Deadline

⚠️ September 16, 2026 — after this date, Node.js 20 will be removed from runners and affected CI will break.