Keep GitHub Actions up to date with GitHub's Dependabot

* [Keeping your software supply chain secure with Dependabot](https://docs.github.com/en/code-security/dependabot)
* [Keeping your actions up to date with Dependabot](https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot)
* [Configuration options for the `dependabot.yml` file - package-ecosystem](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem)