Managed Cloud Service Mesh will start using proxy version csm_mesh_proxy.20260819_RC00 for Gateway API on GKE clusters

## Security Security Managed Cloud Service Mesh will start using proxy version csm\_mesh\_proxy.20260819\_RC00 for Gateway API on GKE clusters. This proxy version maps closest to Envoy version 1.37\. This change is rolling out to all release channels and contains the fix for the managed Cloud Service Mesh security vulnerabilities listed in[GCP-2026-057](https://docs.cloud.google.com/service-mesh/docs/security-bulletins#gcp-2026-057).