FED-1850 Add Dart 2.19 to GHA matrix

[greglittlefield-wf]

Motivation

We're only running GitHub Actions in 2.18, and not 2.19 (the latest minor of Dart 2).

Solution

Add 2.19.6 to the GitHub Actions matrix.

Testing

CI passes, including new 2.19.6 run.

[aviary-wf]

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.

[greglittlefield-wf]

@Workiva/release-management-p