Get Dependabot to also upgrade GitHub Actions

[cclauss]

Keep GitHub Actions up to date with GitHub's Dependabot to clean up the warnings at the bottom right of https://github.com/prestomation/resmed_myair_sensors/actions/runs/5595871057

• https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem

[cclauss]

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