Moving gke used for ci from europe to us (elastic#6889)

This changes the region used to create GKE clusters during the daily e2e tests jobs,
from europe-west6 to us-central1, in order to reduce costs, as there are notable differences
from region to region.

hack/deployer/config/plans.yml

@@ -10,7 +10,7 @@ plans:
   # use kustomize in GKE to remove the NVMe provisioning already taken care of by the platform
   diskSetup: kubectl apply -k hack/deployer/config/local-disks
   gke:
-    region: europe-west6
+    region: us-central1
     localSsdCount: 1
     nodeCountPerZone: 1
     gcpScopes: https://www.googleapis.com/auth/devstorage.read_only,https://www.googleapis.com/auth/logging.write,https://www.googleapis.com/auth/monitoring,https://www.googleapis.com/auth/servicecontrol,https://www.googleapis.com/auth/service.management.readonly,https://www.googleapis.com/auth/trace.append