Improve template CI files

### Pre-flight checklist

- [X] I have read the [contribution documentation](https://github.com/electron-userland/electron-forge/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project uses.
- [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem description

Forge's `npx create-electron-app` script allows you to initialize the repository with a set of templated CI files. Right now, we have configurations for [Travis](https://www.travis-ci.com/) and [AppVeyor](https://www.appveyor.com/).

These templates are fairly old and only cover Linux and Windows operating systems.

### Proposed solution

GitHub Actions has gained traction as a free CI platform for Windows, macOS, and Linux. I propose replacing the existing CI templates with a GitHub Actions workflow that does the same tasks.

### Alternatives considered

* **Update the existing templates**: I'm not a fan of this option because TravisCI has seemed in an unhealthy state for the last few years, and this does not provide macOS support.
* **Use an alternate provider:** [CircleCI](https://circleci.com/) seems to be the other option that provides true cross-platform support. However, its free tier is usage-based, whereas GH Actions is entirely free for public repos.

### Additional information

We should also document this stuff a bit better! I don't think we mention it in the docs. It's only mentioned in the `npx create-electron-app --help` command.