feat!: bump min Dart SDK to 2.19.0

[alestiago]

Description

Related to https://github.com/VeryGoodOpenSource/very_good_cli/issues/702.

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [X] 🗑️ Chore