Update Gardener info (#16031)

- Gardener is a fully managed service, including day-1 and day-2 operations
- Gardener supports OpenStack (and Packet Cloud), which is often the foundation for on-premise installations

See e.g. adopters like https://bnerd.com which are offering a Gardener-based fully managed Kubernetes service in their own OpenStack DCs. More information at https://github.com/gardener/gardener/blob/master/ADOPTERS.md.

content/en/docs/setup/_index.md

@@ -82,7 +82,7 @@ The following production environment solutions table lists the providers and the
 | [Docker Enterprise](https://www.docker.com/products/docker-enterprise) |       |✔  | ✔     |  |     | ✔
 | [Fedora (Multi Node)](https://kubernetes.io/docs/getting-started-guides/fedora/flannel_multi_node_cluster/)     |      |  |      | |    ✔       | ✔
 | [Fedora (Single Node)](https://kubernetes.io/docs/getting-started-guides/fedora/fedora_manual_config/)     |      |  |      | |           | ✔
-| [Gardener](https://gardener.cloud/)       |              |✔  |         | ✔   |          |
+| [Gardener](https://gardener.cloud/) | ✔ | ✔ | ✔ (via OpenStack) | ✔ | |
 | [Giant Swarm](https://giantswarm.io/)     | ✔       | ✔ |   ✔    |     |
 | [Google](https://cloud.google.com/)           |  [Google Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine/)       | [Google Compute Engine (GCE)](https://cloud.google.com/compute/)|[GKE On-Prem](https://cloud.google.com/gke-on-prem/)       |  |  |     |  |      |       |  |
 | [IBM](https://www.ibm.com/in-en/cloud)        | [IBM Cloud Kubernetes Service](https://cloud.ibm.com/kubernetes/catalog/cluster)| |[IBM Cloud Private](https://www.ibm.com/in-en/cloud/private) | |