Upgrading the GKE Version from 1.14 to 1.21

Prombench uses GKE version 1.14 (which is end of life and end of support - https://cloud.google.com/kubernetes-engine/docs/release-schedule).

This commit upgrades the GKE version to 1.21. Although, 1.21 is also end
of life and end of support, it is the latest and greatest version that
can support prombench while not making major changes to the prombench code.
This is because, 1.22 version onwards, the `v1beta` apis have been deprecated
(which prombench uses).

prombench/manifests/cluster_gke.yaml

@@ -2,7 +2,13 @@ projectid: {{ .GKE_PROJECT_ID }}
 zone: {{ .ZONE }}
 cluster:
   name: {{ .CLUSTER_NAME }}
-  initialclusterversion: 1.14
+
+  # Version 1.22+ deprecates the `beta` APIs (https://cloud.google.com/kubernetes-engine/docs/deprecations/apis-1-22)
+  # that are required for prombench to run.
+
+  # v1.14 is EOL: https://cloud.google.com/kubernetes-engine/docs/release-schedule
+  initialclusterversion: 1.21
+
   nodepools:
   # This node-pool will be used for running monitoring components
   - name: main-node