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Noticed the actions used in the workflows here are outdated, proposing a Dependabot configuration to update - reference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 configuration; no application logic changes, though merged Dependabot PRs will still need normal review before they alter workflows.

Overview
Adds .github/dependabot.yml so Dependabot can propose weekly updates to uses: action pins across workflows under the repo root (package-ecosystem: github-actions, directory: /).

This is aimed at keeping workflow actions current and reducing deprecation warnings (e.g. older actions/checkout versions) without changing build or runtime application code.

^{Reviewed by Cursor Bugbot for commit 2cca690. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ScottBrenner]

Hey @archandatta spare a moment to review?