Introduce automation to keep `pnpm/action-setup` up-to-date

Hello! This issue was created semi-automatically because this repo popped up in a search. Per https://infra.apache.org/github-actions-policy.html, 3rd-party actions such as `pnpm/action-setup` must be referred to by hash and kept up-to-date by automation such as dependabot. It looks like this repo does not pollow this policy yet.

We plan to start enforcing this policy for `pnpm/action-setup` through https://github.com/apache/infrastructure-actions/pull/1193 .

If this was a false posivite, you can close this issue. Otherwise, you likely want to update your workflows, since they will stop working once that PR is merged.

Start by inspecting the repository's GitHub Actions workflow files and any existing Dependabot configuration. Check how `pnpm/action-setup` is currently referenced, then compare the setup with the Apache GitHub Actions policy; done means the action is pinned and covered by automation that keeps it up to date.

Written by the indexing model from the issue text.