Update links to deprecated cert integration method for Kubernetes.

Why is this change being made?

Creating clusters from GitLab using cluster certificates is still available on the GitLab UI but was deprecated in GitLab 14.0 and is scheduled for removal in GitLab 15.0. We don’t recommend using this method.

• https://docs.gitlab.com/ee/user/infrastructure/clusters/index.html

We should point to the Kubernetes Agent integration docs now for links related to deploying software from GitLab CI/CD pipelines to Kubernetes.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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