chore: add dependabot auto-merge workflow - #34

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: devcontainers/action@1082abd5d2bf3a11abccba70eef98df068277772. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/