Add GitHub Actions to Dependabot

[petergoldstein]

This PR adds Dependabot for GitHub Actions as described here - https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

As the checkout action is currently out of date, Dependabot will open a PR to update it if and when this PR is merged.