Google Ads API Client Library for PHP v31.0.0

• Compatibility with v21.0 of the API: https://developers.google.com/google-ads/api/docs/release-notes
• Added support for declaring EU political advertising status on campaign creation in examples.
• Pinned the 'grpc/grpc' dependency so it doesn't get ahead of the C 'ext-grpc'.
• Removed support for v18 of the API: https://ads-developers.googleblog.com/2025/07/google-ads-api-v18-sunset-reminder.html