FEDX-1401: Added dependentabot for gha

[matthewnitschke-wk]

FEDX-1401

We want to monitor gha dependencies and keep them up to date

This change will also test: https://github.com/Workiva/gha-dart-oss/pull/13

[aviary3-wk]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure

• Verify aviary.yaml coverage of security relevant code

  Questions or Comments? Reach out on Slack: #support-infosec.