build(github-actions): add workflow to validate dependabot config

[fengelniederhammer]

PR Checklist

• [ ] All necessary documentation has been adapted.
• [ ] The implemented feature is covered by an appropriate test.

[github-actions[bot]]

🚫 dependabot errors

keyword	message	dataPath
additionalProperties	should NOT have additional properties

[fengelniederhammer]

GitHub already seems to have an automatic check for this that's even better than that workflow.