chore: remove support for GKE 1.20

ref: https://cloud.google.com/kubernetes-engine/docs/release-notes-nochannel#October_05_2022

deploy/README.md

@@ -81,7 +81,7 @@ The following table displays the tested Kubernetes and Helm versions.
 |---------------|------------------------------------------|
 | K8s with EKS  | 1.19<br/>1.20<br/>1.21<br/>1.22<br/>1.23 |
 | K8s with Kops | 1.20<br/>1.21<br/>1.22<br/>1.23<br/>1.24 |
-| K8s with GKE  | 1.20<br/>1.21<br/>1.22<br/>1.23          |
+| K8s with GKE  |1.21<br/>1.22<br/>1.23          |
 | K8s with AKS  | 1.22<br/>1.23<br/>1.24                   |
 | OpenShift     | 4.6<br/>4.7<br/>4.8<br/>4.9<br/>4.10     |
 | Helm          | 3.8.2 (Linux)                            |