feat: promote config connector gke addon to ga

[modular-magician]

promotes change from #6300 to GA.

as far as I can tell, the config connector gke add on is no longer in beta, and thus should be made available on the GA terraform provider.

ref:

• mentioned in release notes https://cloud.google.com/kubernetes-engine/docs/release-notes#January_29_2020
• included in the stable gcloud create command https://cloud.google.com/sdk/gcloud/reference/container/clusters/create#--addons
• no beta messaging on https://cloud.google.com/config-connector/docs/how-to/install-upgrade-uninstall

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). There are no open issues, happy to open one if needed but seems like a small change
• 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).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: promoted config connector addon to GA

Derived from GoogleCloudPlatform/magic-modules#6976

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