move multi-network to GA

[kkuchima]

This PR is to add terraform support for Multi-network GA on GKE cluster.

The Multi-network in Terraform beta was added to the magic module in the PR #8385.
This PR basically moves the enable_multi_networking field in google_container_cluster and additional_node_network_configs additional_pod_network_configs fields in google_container_node_pool to GA.

Release Note Template for Downstream PRs (will be copied)

container: added multi-NIC network for GKE cluster and node pool.