bug(misconf): false positive GCP-0076 for proxy subnetwork

[nikpivkin]

GCP-0076 reports a false positive when scanning proxy-only GCP subnetworks.
Since flow logs are not supported for proxy-only subnetworks, the check should not require them to be enabled.

Discussed in #10188

[vsaraths]

GCP-0076 currently flags proxy-only subnetworks when flow logs are not enabled.

However, since proxy-only subnetworks do not support VPC flow logs by design, this results in a false positive during misconfiguration scanning.

Skipping validation for proxy-only subnetworks is the correct behavior and keeps the check aligned with GCP’s actual capabilities.

[nikpivkin]

Resolved in aquasecurity/trivy-checks#529