Istio 1.31 Adds Agentgateway Waypoints and Moves Release Artifacts off Google Cloud

Istio 1.31 adds agentgateway waypoints in ambient mode, with a canary configuration fix included in 1.31.1. It also ends the publication of images and Helm charts to Google Cloud, requiring repository migration ahead of the 13 October outage test and signing-key updates for teams verifying images.