Update google_dns_managed_zone.private_visibility_config to require at least one of gke_clusters or networks

[modular-magician]

#15428

With cluster-scoped dns zones (https://cloud.google.com/dns/docs/zones/configure-scopes), you can now specify a zone that applies just to a gke cluster, without applying to the entire network. This change removes the required attribute of private_visibility_config.networks field and sets at_least_one_of on the private_visibility_config to allow for one or both of gke_clusters or networks.

gcloud output confirms this:

$ gcloud dns managed-zones create foo --dns-name=foo. --visibility=private --description=foo

ERROR: (gcloud.dns.managed-zones.create) Missing required argument [--networks, --gkeclusters]: If --visibility is set to private, a list of networks or list of GKE clusters must be provided.'  NOTE: You can provide an empty value ("") for private zones that have NO network or GKE clusters binding.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

DNS: changed `private_visibility_config.networks` from `required` to requiring at least one of `private_visibility_config.networks` or `private_visibility_config.gke_clusters` in `google_dns_managed_zone`

Derived from GoogleCloudPlatform/magic-modules#8581

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