fix: compute instance internal ipv6 prefix length is not set in creation and read in syncing

[jingjia-peng]

Fixes hashicorp/terraform-provider-google#21520.

This ensures that Terraform correctly reads the compute_instance.network_interface.internal_ipv6_prefix_length from GCP and stores it in terraform.state. Previously, the absence of this value in terraform.state caused compute_instance to be considered modified, even when no changes were made to the configuration.

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compute: fix `compute_instance.network_interface.internal_ipv6_prefix_length` not being set or read in Terraform state  

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