fix(gcp): update Ubuntu 22.04 GCP image to v20260313

The previous image (ubuntu-2204-jammy-v20240904) has stale apt metadata
that causes GPG signature verification failures on apt-get update,
breaking fakeintake VM provisioning in E2E tests.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: fanny-jiang

scenarios/gcp/compute/os_resolver.go

@@ -19,7 +19,7 @@ func resolveUbuntuImage(_ gcp.Environment, osInfo os.Descriptor) (string, error)
 
 	switch osInfo.Version {
 	case os.Ubuntu2204.Version:
-		return "ubuntu-2204-jammy-v20240904", nil
+		return "ubuntu-2204-jammy-v20260313", nil
 	default:
 		return "", nil
 	}