Add support for Kubernetes version 1.20
## Notes
This is an umbrella issue primarily for the release note as we tackle it together for Kubernetes 1.20
## Release notes
https://docs.gitlab.com/ee/user/project/clusters/#supported-cluster-versions
By adding support to more recent Kubernetes versions GitLab users can benefit from GitLab's various Kubernetes integrations, like the [GitLab Kubernetes Agent](https://docs.gitlab.com/ee/user/clusters/agent/), [Auto DevOps](https://docs.gitlab.com/ee/topics/autodevops/) and [Cluster Management](https://docs.gitlab.com/ee/user/clusters/management_project.html) even on more recent clusters. Previously official support was provided only up to Kubernetes version 1.19. We have added support for Kubernetes 1.20 now.
You can find the list of [supported versions and related timelines in our documentation](https://docs.gitlab.com/ee/user/project/clusters/#supported-cluster-versions).
## Related issues
- https://gitlab.com/gitlab-org/gitlab/-/issues/212589
- https://gitlab.com/gitlab-org/gitlab/-/issues/292258
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