[Platform][Kubernetes] Use az.updateConfig for incluster credentials

c0b9786 changed the behavior of
az.setConfig and added az.updateConfig. This change uses the correct
method to add `{"KUBECONFIG": ""}` to the existing configuration
instead of overwriting the whole AZ configuration.

This was missed when I rebased
yugabyte#8371 against latest
changes.

Fixes yugabyte#10860

Signed-off-by: Bhavin Gandhi <bhavin7392@gmail.com>

managed/src/main/java/com/yugabyte/yw/controllers/handlers/CloudProviderHandler.java

@@ -219,7 +219,7 @@ public Provider createKubernetes(Customer customer, KubernetesProviderFormData f
         boolean isConfigInZone = updateKubeConfigForZone(provider, region, az, zoneConfig, false);
         if (!(isConfigInProvider || isConfigInRegion || isConfigInZone)) {
           // Use in-cluster ServiceAccount credentials
-          az.setConfig(ImmutableMap.of("KUBECONFIG", ""));
+          az.updateConfig(ImmutableMap.of("KUBECONFIG", ""));
           az.save();
         }
       }
@@ -287,7 +287,7 @@ public Provider createKubernetesNew(Customer customer, Provider reqProvider) thr
         boolean isConfigInZone = updateKubeConfigForZone(provider, region, az, zoneConfig, false);
         if (!(isConfigInProvider || isConfigInRegion || isConfigInZone)) {
           // Use in-cluster ServiceAccount credentials
-          az.setConfig(ImmutableMap.of("KUBECONFIG", ""));
+          az.updateConfig(ImmutableMap.of("KUBECONFIG", ""));
           az.save();
         }
       }