ci: add dependabot config for GitHub Actions and npm dependencies - #97

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/dependency-review-action@da24556b548a50705dd671f47852072ea4c105d9, step-security/harden-runner@ec9f2d5744a09debf3a187a3f4f675c53b671911. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/