Add Dependabot for GitHub Actions

[petergoldstein]

What is the current behavior?

GitHub Actions in CI configuration are not updated as new versions are made available.

What is the new behavior?

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As the checkout action is currently out of date, Dependabot will open a PR to update it if and when this PR is merged.

Checklist

Please make sure the following requirements are complete:

• Tests for the changes have been added (for bug fixes / features) - N/A for CI configuration changes
• Docs have been reviewed and added / updated if needed (for bug fixes / features) - N/A for CI configuration changes
• [x] All automated checks pass (CI/CD)