added default value for minPortsPerVM field at "google_compute_router_nat"

[modular-magician]

Fixes: #2977

Fixes the recreation issue for the google_compute_router_nat resource on sucesive terraform apply

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• 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.
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Release Note Template for Downstream PRs (will be copied)

compute: removed diff detection on `min_ports_per_vm` in `google_compute_router_nat` when the field is unset

Derived from GoogleCloudPlatform/magic-modules#9712

[github-actions]

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