change enable_shielded_nodes default to true on google_container_cluster

[ScottSuarez]

Currently we have set container_node_cluster default to force send false due to computed/optional issue. This is suboptimal, but changing to default true and non-computed would be a breaking change. Since shielded config is useful. Lets consider for 4.0.0

References

• enable_shielded_nodes is not sent for container cluster resource #9118

[github-actions]

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