dependabot: add Terraform for user-management repo

[p-linnane]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Adding Terraform so that we can support updating dependencies in the user-management repo. While here, I also alphabetized the ecosystems for clarity.

[MikeMcQuaid]

# This file is synced from the .github repository, do not modify it directly.

Modify template there instead 🙇🏻

You'll also need to adjust the logic accordingly too.

[p-linnane]

🤦 Whoops. Thanks for catching that. Will handle over there.

[p-linnane]

Homebrew/.github#339