Fix build on ios

[mario-bermonti]

Description

1. Upgrade ios target version to 12+
2. Ignore "unused" flags (e.g., -G) - see discussion fix: Remove unused -GCC_WARN_INHIBIT_ALL_WARNINGS flag grpc/grpc#36904

Related Issue

Closes #86

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🐞 Bug fix (non-breaking change that fixes an issue)
• 🔧 Maintenance (non-breaking change that improves code)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix
• 🔐 Improvements to the CI workflow

Checklist

• I've read the project's code of conduct.
• I've read the contributing guide.
• I've written tests for all new methods and classes that I created.