[ci] Add dependabot configuration

[borchero]

Motivation

Follow-up to #6438 (comment). This should automate updates in the future by opening <= 1 PR per month updating all relevant actions in bulk.

[jameslamb]

Awesome 🎉

I read through https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file today (I've never actually configured this on a repo before) and agree with all the choices you made here.

And I was very happy to learn (from https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates) that you can enable this just by checking in a file, without having to grant any permission to a GitHub App in the repo settings.

Let's try it!

[github-actions]

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