fix: bump grpc-gcp-go to v1.6.0 for grpc v1.80.0 compatibility

grpc-gcp-go v1.5.2 doesn't implement ExitIdle required by grpc
v1.80.0 balancer interface, causing build failures.

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

Author: buger

go.mod

@@ -168,7 +168,7 @@ require (
 	github.com/ClickHouse/ch-go v0.66.0 // indirect
 	github.com/ClickHouse/clickhouse-go/v2 v2.36.0 // indirect
 	github.com/DataDog/zstd v1.5.6 // indirect
-	github.com/GoogleCloudPlatform/grpc-gcp-go/grpcgcp v1.5.2 // indirect
+	github.com/GoogleCloudPlatform/grpc-gcp-go/grpcgcp v1.6.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.25.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/trace v1.21.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/internal/resourcemapping v0.45.0 // indirect