Fix network field in google_compute_region_backend_service to force new on change (#15136)

Author: SCSAndre

mmv1/products/compute/RegionBackendService.yaml

@@ -1439,10 +1439,13 @@ properties:
     type: ResourceRef
     description: |
       The URL of the network to which this backend service belongs.
-      This field can only be specified when the load balancing scheme is set to INTERNAL.
+      This field must be set for Internal Passthrough Network Load Balancers when the haPolicy is enabled, and for External Passthrough Network Load Balancers when the haPolicy fastIpMove is enabled.
+      This field can only be specified when the load balancing scheme is set to INTERNAL, or when the load balancing scheme is set to EXTERNAL and haPolicy fastIpMove is enabled.
+      Changes to this field force recreation of the resource.
     custom_expand: 'templates/terraform/custom_expand/resourceref_with_validation.go.tmpl'
     resource: 'Network'
     imports: 'selfLink'
+    immutable: true
   - name: 'subsetting'
     type: NestedObject
     description: |