Support Node.js 24 to keep this action working

### Describe the problem

This action will stop working soon - my workflows that use it have started issuing this warning:

```
Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: arduino/setup-protoc@c65c819552d16ad3c9b72d9dfd5ba5237b9c906b. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/
```

Probably time to upgrade?

Thanks for maintaining this action!

### To reproduce

Run the action on a GitHub project

### Expected behavior

It'd use Node.js 24

### 'arduino/setup-protoc' version

c65c819552d16ad3c9b72d9dfd5ba5237b9c906b

### Additional context

https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

### Issue checklist

- [x] I searched for previous reports in [the issue tracker](https://github.com/arduino/setup-protoc/issues?q=)
- [x] I verified the problem still occurs when using the latest version
- [x] My report contains all necessary details

Start by reviewing open pull request #113 and the action's runtime metadata, since the issue does not name a file or test. Confirm the action is configured for Node.js 24, then run it in a GitHub project and verify the Node.js 20 deprecation warning is gone.

Written by the indexing model from the issue text.