Keep GitHub Actions up to date with GitHub's Dependabot

Keep GitHub Actions up to date with GitHub's Dependabot

Fixes software supply chain safety warnings like at the bottom right of

https://github.com/fairy-stockfish/Fairy-Stockfish/actions/runs/12274061623
* [Keeping your actions up to date with Dependabot](https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot)
* [Configuration options for the dependabot.yml file - package-ecosystem](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem)