Dependabot: auto-update GitHub Actions

[sgerrand]

💁 These changes configure Dependabot to automatically update the versions of any GitHub Actions that are used in existing workflows. Using this bot should prevent errors like this one in #281 and improve the maintainability of this project:

Error: This request has been automatically failed because it uses a deprecated version of `actions/cache: v2`. 
Please update your workflow to use v3/v4 of actions/cache to avoid interruptions. 
Learn more: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

[sgerrand]

👋 @davissp14, would you please review this changeset when you get a chance?

[sgerrand]

👋 @davissp14, would you please review this change when you have a moment.