Add Dependabot ignore directives

[mcdonnnj]

🗣 Description

This pull request adds commented out ignore directives for some of the GitHub Actions we rely on for our workflows. In downstream repositories they should be uncommented so that updates to the following Actions flow from this skeleton:

• actions/cache
• actions/checkout
• actions/setup-python

💭 Motivation and context

This is similar to the work in https://github.com/cisagov/skeleton-packer/pull/105, https://github.com/cisagov/skeleton-ansible-role/pull/97, and https://github.com/cisagov/skeleton-ansible-role-with-test-user/pull/42.

🧪 Testing

Automated tests pass. I ensure that in a clone repository the ignore directives closed pull requests opened by Dependabot as expected.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.