Add plusserver

Author: sbernauer

modules/ROOT/partials/tested-kubernetes-distributions.adoc

@@ -1,5 +1,6 @@
 * xref:kubernetes/huawei-cloud.adoc[Huawei Cloud]
 * xref:kubernetes/ibm-cloud.adoc[IBM Cloud]
+* https://www.plusserver.com/en/product/managed-kubernetes/[plusserver Kubernetes as a Service]
 * xref:kubernetes/ske.adoc[STACKIT Kubernetes Engine] (with the exception of missing public NodePorts)
 * xref:kubernetes/vmware_tanzu.adoc[VMware Tanzu]
 * https://www.oracle.com/cloud/cloud-native/kubernetes-engine/[Oracle Kubernetes Engine (OKE)]