Update pub get commands to dart pub get

[evanweible-wf]

Motivation

Newer versions of Dart don't support the pub executable on its own, it must be run via dart pub.

Changes

Update code that runs pub get subprocesses to run dart pub get instead.

[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

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[robbecker-wf]

QA+1 CI passes @Workiva/release-management-p

[rmconsole-wf]

+1 from RM