Add Dependabot configuration for GitHub Actions

[ScottBrenner]

Noticed the actions used in the workflows here are outdated, proposing a Dependabot configuration to update - reference https://docs.github.com/en/actions/security-guides/using-githubs-security-features-to-secure-your-use-of-github-actions#keeping-the-actions-in-your-workflows-secure-and-up-to-date

Resolves warnings on workflow runs e.g. https://github.com/kernel/kernel-images/actions/runs/30600123889

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4

Suggest enabling https://docs.github.com/en/code-security/dependabot/working-with-dependabot/about-dependabot-on-github-actions-runners#enabling-or-disabling-for-your-repository as well

Checklist

• A link to a related issue in our repository
• A description of the changes proposed in the pull request.
• @mentions of the person or team responsible for reviewing proposed changes.

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Note

Low Risk
CI-only config with no application, auth, or data-handling changes.

Overview
Adds a Dependabot config so GitHub Actions in the repo root are checked for updates weekly, helping keep workflow actions current.

^{Reviewed by Cursor Bugbot for commit 03dc14c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ScottBrenner]

Hey @masnwilliams spare a moment to review?