ci: add Dependabot auto-merge workflow - #6

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. This request has been automatically failed because it uses a deprecated version of `actions/upload-artifact: v3`. Learn more: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/