Transition between Kubernetes integration service and Clusters

This meta is intended to track the transition between Kubernetes integration service and Clusters https://gitlab.com/gitlab-org/gitlab-ce/issues/38668#note_49103753

1. Remove Kubernetes Integration from Project
2. Leave Kubernetes Integration as Application Template to populate CI/CD > Clusters
3. Make KubernetesService hold template parameters only, not logic

10.3 -

• Announce removal of Kubernetes Integration from Project

10.4 -

• Disable creation of new Kubernetes Integrations unless they are active and created from template: https://gitlab.com/gitlab-org/gitlab-ce/issues/41054
• Migrate Kubernetes Integration data to Clusters::Cluster: https://gitlab.com/gitlab-org/gitlab-ce/issues/40418
• Extend deployment_platform to lazily create Clusters::Cluster from Kubernetes Application Template: https://gitlab.com/gitlab-org/gitlab-ce/issues/41056

10.5 -

• Make Kubernetes Service to be a shallow object to hold only parameters, not logic: https://gitlab.com/gitlab-org/gitlab-ce/issues/41413
• Remove Kubernetes Integration page: https://gitlab.com/gitlab-org/gitlab-ce/issues/39217