Add deprecation warnings for VariantStock methods

This will prevent other devs from relying on this methods and will tell
us the exact lines that call this methods, which will come in handy when
removing this module.

lib/open_food_network/variant_stock.rb

@@ -27,6 +27,8 @@ module OpenFoodNetwork
     #
     # @return [Float|Integer]
     def on_hand
+      warn_deprecation(__method__, '#total_on_hand')
+
       if on_demand
         Float::INFINITY
       else
@@ -38,6 +40,7 @@ module OpenFoodNetwork
     #
     # @return [Float|Integer]
     def count_on_hand
+      warn_deprecation(__method__, '#total_on_hand')
       total_on_hand
     end
 
@@ -47,6 +50,8 @@ module OpenFoodNetwork
     # @raise [StandardError] when the track_inventory_levels config
     # key is not set.
     def on_hand=(new_level)
+      warn_deprecation(__method__, '#total_on_hand')
+
       error = 'Cannot set on_hand value when Spree::Config[:track_inventory_levels] is false'
       raise error unless Spree::Config.track_inventory_levels
 
@@ -67,6 +72,8 @@ module OpenFoodNetwork
     #
     # @raise [StandardError] when the variant has no stock item yet
     def count_on_hand=(new_level)
+      warn_deprecation(__method__, '#total_on_hand')
+
       raise_error_if_no_stock_item_available
       overwrite_stock_levels(new_level)
     end
@@ -77,7 +84,7 @@ module OpenFoodNetwork
     # track_inventory_levels only. It was initially introduced in
     # https://github.com/openfoodfoundation/spree/commit/20b5ad9835dca7f41a40ad16c7b45f987eea6dcc
     def on_demand
-      warn_deprecation(__method__)
+      warn_deprecation(__method__, 'Spree::Config[:track_inventory_levels]')
       stock_items.first.backorderable?
     end
 
@@ -88,6 +95,8 @@ module OpenFoodNetwork
     #
     # @raise [StandardError] when the variant has no stock item yet
     def on_demand=(new_value)
+      warn_deprecation(__method__, 'Spree::Config[:track_inventory_levels]')
+
       raise_error_if_no_stock_item_available
 
       # There should be only one at the default stock location.
@@ -125,5 +134,12 @@ module OpenFoodNetwork
       # have only one stock location.
       stock_items.first.__send__(:count_on_hand=, new_level)
     end
+
+    def warn_deprecation(method_name, new_method_name)
+      ActiveSupport::Deprecation.warn(
+        "`##{method_name}` is deprecated and will be removed. " \
+        "Please use `#{new_method_name}` instead."
+      )
+    end
   end
 end