update dart classifier to support 2.x and arm/v7 in 3.x

[witchcraze]

Description

• Fixes Support Dart arm/v7 in 3.x and 2.x #3278

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have added unit tests that cover changed behavior
• I have tested my code in common scenarios and confirmed there are no regressions
• I have added comments to my code, particularly in hard-to-understand sections

note:
As make add-snippet wrote snippet to "classifiers/snippets/dart/3.0.0/linux-unknown-454d5f41524d/dart", I renamed folder to match under classifiers/bin/dart/3.0.0

---

$ go run cmd/syft/main.go -q --platform linux/arm/v7 dart:3.0.0-sdk | grep dart
dart                    3.0.0                           binary

$ go run cmd/syft/main.go -q dart:2 | grep dart
dart                    2.19.6                          binary

[kzantow]

👍