FEDX-519 Raise maximum SDK to include Dart 3 and update CI

[evanweible-wf]

This package is already compatible with Dart 3. This PR raises the maximum SDK to <4.0.0 to make that explicit. It also updates CI to run on 2.19 and stable.

[aviary2-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.

[evanweible-wf]

QA +1

• [x] CI passes on Dart 2.19 and stable on both ubuntu and windows
• [x] CI runs include analysis, dep validation, and unit tests on all combinations of the matrix
• [x] CI runs formatting check only on Dart stable on ubuntu

@Workiva/release-management-p