ci(dependabot): update GitHub Actions weekly

[caugner]

Description

Adds or updates the Dependabot config for GitHub Actions:

  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"
    commit-message:
      prefix: "ci(deps): "

Motivation

Ensures used GitHub Action dependencies are updated consistently and regularly (but not daily) across our repos.

Additional details

See also: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference

Related issues and pull requests

Part of mdn/fred#887.