Update dependabot workflow

[sosnovsky]

There are such warnings regarding our dependabot workflow, need to check how it should be updated to continue working correctly:

Workflow execution policy warning (evaluate mode): .github/workflows/auto-merge.yml#L1
On November 2, 2026, GitHub will restrict pull_request_target on public repositories by default. To continue allowing the event trigger, configure an Actions policy. Learn more: https://gh.io/securely-using-pull_request_target#default-policy-for-pull_request_target

Enable automerge on dependabot PRs
Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: daneden/enable-automerge-action@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/