Add Dependabot for GitHub Actions and pin workflow actions - #80

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, leafo/gh-actions-lua@35bcb06abec04ec87df82e08caa84d545348536e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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