fix networking_mode in container_cluster

[modular-magician]

networking_mode was incorrectly being set to VPC_NATIVE on
routes-based clusters because the return value in
expandIPAllocationPolicy was not setting use_routes to true. This was causing one of KCC's tests to fail because networking_mode was changing when it is an immutable field.

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Release Note Template for Downstream PRs (will be copied)

container: fixed the `ROUTES` value for the `networking_mode` field in `google_container_cluster`. A recent API change unintentionally changed the default to a `VPC_NATIVE` cluster, and removed the ability to create a `ROUTES`-based one. Provider versions prior to this one will default to `VPC_NATIVE` due to this change, and are unable to create `ROUTES` clusters.

Derived from GoogleCloudPlatform/magic-modules#5504