fix: referring to mix package key as hex.

[zkoppert]

fixes #70

Pull Request

Proposed Changes

This pull request includes changes to the dependabot_file.py and test_dependabot_file.py files, specifically in the build_dependabot_file function and the test_build_dependabot_file_with_hex test case. The changes are focused on replacing the package ecosystem from 'hex' to 'mix'.

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run make lint and fix any issues that you have introduced
• run make test and ensure you have test coverage for the lines you are introducing

Reviewer

• Label as either bug, documentation, enhancement, infrastructure, or breaking