chore: restore 1 required approval on main after the parity milestone #14

Part of #2. For a human. Not for the factory.

For the parity milestone, main's branch protection was relaxed from 1 required approving review to 0 (enforce_admins stays on), because factory PRs are authored from @erikist's account, which can't approve its own PRs. This is the same arrangement as stablekernel/protoc-gen-go-mcp#93.

When the milestone's PRs have merged, restore it:

gh api -X PATCH repos/stablekernel/protoc-gen-rust-mcp/branches/main/protection/required_pull_request_reviews \
  -F required_approving_review_count=1 -F dismiss_stale_reviews=true