chore: bump flutter/dart min sdk constraints

[btrautmann]

Description

Bumps minimum version constraint on Flutter/dart to 3.16.0/3.2.0 respectively. Doing this ahead of https://github.com/Betterment/alchemist/pull/115 should resolve the issues seen with the build there.

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

[btrautmann]

Blocked by the issue(s) mentioned at https://github.com/Betterment/alchemist/issues/104 and https://github.com/Betterment/alchemist/issues/103, looking into it...

[btrautmann]

/no-platform