fix networking_mode in container_cluster (#5504) (#10686)

Co-authored-by: Emily Cai <[email protected]>
Signed-off-by: Modular Magician <[email protected]>

Co-authored-by: Emily Cai <[email protected]>

Author: modular-magician

.changelog/5504.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+container: fixed the `ROUTES` value for the `networking_mode` field in `google_container_cluster`. A recent API change unintentionally changed the default to a `VPC_NATIVE` cluster, and removed the ability to create a `ROUTES`-based one. Provider versions prior to this one will default to `VPC_NATIVE` due to this change, and are unable to create `ROUTES` clusters.
+```

google/resource_container_cluster.go

@@ -2553,6 +2553,7 @@ func expandIPAllocationPolicy(configured interface{}, networkingMode string) (*c
 		}
 		return &container.IPAllocationPolicy{
 			UseIpAliases:    false,
+			UseRoutes:       true,
 			ForceSendFields: []string{"UseIpAliases"},
 		}, nil
 	}

website/docs/r/container_cluster.html.markdown

@@ -180,7 +180,7 @@ VPC-native clusters. Adding this block enables [IP aliasing](https://cloud.googl
 making the cluster VPC-native instead of routes-based. Structure is [documented
 below](#nested_ip_allocation_policy).
 
-* `networking_mode` - (Optional, [Beta]) Determines whether alias IPs or routes will be used for pod IPs in the cluster.
+* `networking_mode` - (Optional) Determines whether alias IPs or routes will be used for pod IPs in the cluster.
 Options are `VPC_NATIVE` or `ROUTES`. `VPC_NATIVE` enables [IP aliasing](https://cloud.google.com/kubernetes-engine/docs/how-to/ip-aliases),
 and requires the `ip_allocation_policy` block to be defined. By default when this field is unspecified, GKE will create a `ROUTES`-based cluster.