Remove Flutter version pin

[domesticmouse]

Description

Remove Flutter version pin to allow the sample to be run on current Flutter versions.

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
• [x] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

[domesticmouse]

PTAL @bselwe