Enable Terraform as a `package-ecosystem` in the Dependabot Configuration

[mcdonnnj]

💡 Summary

Add terraform as a package-ecosystem in the Dependabot configuration once we have fully migrated to Terraform 0.13 (or newer).

Motivation and context

With the migration to Terraform 0.13 (and beyond!) we can leverage Dependabot scanning per the documentation as version 0.13 is the minimum required.

Implementation notes

This same change should be applied anywhere we use Terraform code regularly such as for projects descending from cisagov/skeleton-packer and cisagov/skeleton-ansible-role-with-test-user.

Acceptance criteria

• There is a package-ecosystem entry for terraform in the Dependabot configuration