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

# 💡 Summary #

Add `terraform` as a `package-ecosystem` in the [Dependabot configuration](https://github.com/cisagov/skeleton-tf-module/blob/develop/.github/dependabot.yml) 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](https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/configuration-options-for-dependency-updates#package-ecosystem) 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](https://github.com/cisagov/skeleton-packer) and [cisagov/skeleton-ansible-role-with-test-user](https://github.com/cisagov/skeleton-ansible-role-with-test-user).

## Acceptance criteria ##

- [ ] There is a `package-ecosystem` entry for `terraform` in the [Dependabot configuration](https://github.com/cisagov/skeleton-tf-module/blob/develop/.github/dependabot.yml)

Start by checking whether this repository has fully migrated to Terraform 0.13 or newer, then inspect .github/dependabot.yml. Add the Terraform package-ecosystem entry once the prerequisite is met, and check whether the related skeleton projects also use Terraform regularly. Done means the configuration contains the requested entry and the acceptance criterion is satisfied.

Written by the indexing model from the issue text.