update the adoption status of containerd in GKE

ADOPTERS.md

@@ -12,7 +12,7 @@ including the Balena project listed below.
 
 **_[IBM Cloud Private (ICP)](https://www.ibm.com/cloud/private)_** - IBM's on-premises cloud offering has containerd as a "tech preview" CRI runtime for the Kubernetes offered within this product for the past two releases, and plans to fully migrate to containerd in a future release.
 
-**_[Google Cloud Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine/)_** - offers containerd as the CRI runtime in **beta** for recent versions of Kubernetes.
+**_[Google Cloud Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine/)_** - containerd has been offered in GKE since version 1.14 and has been the default runtime since version 1.19. [More details](https://cloud.google.com/kubernetes-engine/docs/concepts/using-containerd)
 
 **_[AWS Fargate](https://aws.amazon.com/fargate)_** - uses containerd + Firecracker (noted below) as the runtime and isolation technology for containers run in the Fargate platform. Fargate is a serverless, container-native compute offering from Amazon Web Services.