Operator Install guides for AKS and GKE use outdated K8s versions

Looking at the staging docs for 2.0.3 and 2.1, they recommend using older versions of Kubernetes; 

https://docs-staging.couchbase.com/operator/2.0/tutorial-gke.html recommends 1.14 (which will cause the clusters create command to error), and https://docs-staging.couchbase.com/operator/2.1/tutorial-aks.html recommends 1.12.

Both currently seem to support, at the lowest, 1.15:

[https://docs.microsoft.com/en-us/azure/aks/supported-kubernetes-versions
] https://cloud.google.com/kubernetes-engine/docs/release-notes