Add dependabot config for github-actions ecosystem

[jbampton]

Basic setup added to automate updates

https://docs.github.com/en/code-security/dependabot/ecosystems-supported-by-dependabot/supported-ecosystems-and-repositories

Changes

How I tested this

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.