Add deprecation warning when `differencing_disk` option is used

plugins/providers/hyperv/config.rb

@@ -65,6 +65,9 @@ module VagrantPlugins
       end
 
       def finalize!
+        if @differencing_disk != UNSET_VALUE
+          @_differencing_disk_deprecation = true
+        end
         @linked_clone = false if @linked_clone == UNSET_VALUE
         @differencing_disk = false if @differencing_disk == UNSET_VALUE
         @linked_clone ||= @differencing_disk
@@ -92,6 +95,10 @@ module VagrantPlugins
       def validate(machine)
         errors = _detected_errors
 
+        if @_differencing_disk_deprecation && machine
+          machine.ui.warn I18n.t("vagrant_hyperv.config.differencing_disk_deprecation")
+        end
+
         if !vm_integration_services.is_a?(Hash)
           errors << I18n.t("vagrant_hyperv.config.invalid_integration_services_type",
             received: vm_integration_services.class)

templates/locales/providers_hyperv.yml

@@ -35,6 +35,10 @@ en:
 
           Received: %{entry_value}
           Allowed: true, false
+      differencing_disk_deprecation: |-
+        The `differencing_disk` configuration option is deprecated and should
+        no longer be used. The `linked_clone` configuration option should
+        be used instead.
     errors:
       admin_required: |-
         The Hyper-V provider requires that Vagrant be run with