Promote node system config to GA

[modular-magician]

Node system config GA since 2021-02-22 [1]

[1] https://cloud.google.com/kubernetes-engine/docs/release-notes#February_22_2021

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).

  Created an issue here: Promote node system config to GA #13257

• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.

• 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).

• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
  Don't have a project to run them against

• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: promoted node system config in `google_container_node_pool` to GA

Derived from GoogleCloudPlatform/magic-modules#6975

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