Update stellarwp/schema to v3

[dpanta94]

Summary

Updates stellarwp/schema to v3.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• ⚡ Performance improvement
• ♻️ Code refactoring
• 🧪 Test improvements
• 🔧 Chore (build process, dependencies, etc.)

Related Issues

Closes #

Pre-Submission Checklist

Code Quality

• PHPStan analysis passes: composer test:analysis
• PHP compatibility check passes: composer compatibility
• Coding standards pass: vendor/bin/phpcs
• All tests pass: slic run wpunit && slic run integration
• No debug code (var_dump, error_log, etc.) left in production code
• No commented-out code unless specifically needed for reference
• Documentation updated for any new features or changed behavior
• New tests have been added for new functionality
• All existing tests continue to pass

📖 Read the full contributing guidelines: CONTRIBUTING.md

[lucatume]

Smaller readability changes, otherwise looks good to me.