fix(shorebird_cli): prune vended flutter branches on upgrade

[bryanoltman]

Description

This is intended to fix an issue caused by our recent remote branch change:

$ shorebird --version
Updating Flutter...
error: cannot lock ref 'refs/remotes/origin/shorebird/main': 'refs/remotes/origin/shorebird' exists; cannot create 'refs/remotes/origin/shorebird/main'
From https://github.com/shorebirdtech/flutter
 ! [new branch]          shorebird/main -> origin/shorebird/main  (unable to update local ref)

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
• 🗑️ Chore