Deprecate @payment_sources ivar from checkout payments controller

This method returns payment sources of the current user. It is no more
used into our frontend. We used to assign the content of this method
into an ivar (@payment_sources) into the checkout payment step. This
method is here only to be able to deprecate this ivar and will be removed.

frontend/app/controllers/spree/checkout_controller.rb

@@ -219,9 +219,14 @@ def before_payment
         @wallet_payment_sources = try_spree_current_user.wallet.wallet_payment_sources
         @default_wallet_payment_source = @wallet_payment_sources.detect(&:default) ||
                                          @wallet_payment_sources.first
-        # TODO: How can we deprecate this instance variable?  We could try
-        # wrapping it in a delegating object that produces deprecation warnings.
-        @payment_sources = try_spree_current_user.wallet.wallet_payment_sources.map(&:payment_source).select { |ps| ps.is_a?(Spree::CreditCard) }
+
+        @payment_sources = Spree::DeprecatedInstanceVariableProxy.new(
+          self,
+          :deprecated_payment_sources,
+          :@payment_sources,
+          Spree::Deprecation,
+          "Please, do not use @payment_sources anymore, use @wallet_payment_sources instead."
+        )
       end
     end
 
@@ -251,5 +256,22 @@ def insufficient_stock_error
         end
       end
     end
+
+    # This method returns payment sources of the current user. It is no more
+    # used into our frontend. We used to assign the content of this method
+    # into an ivar (@payment_sources) into the checkout payment step. This
+    # method is here only to be able to deprecate this ivar and will be removed.
+    #
+    # DO NOT USE THIS METHOD!
+    #
+    # @return [Array<Spree::PaymentSource>] Payment sources connected to
+    #   current user wallet.
+    # @deprecated This method has been added to deprecate @payment_sources
+    #   ivar and will be removed. Use @wallet_payment_sources instead.
+    def deprecated_payment_sources
+      try_spree_current_user.wallet.wallet_payment_sources
+        .map(&:payment_source)
+        .select { |ps| ps.is_a?(Spree::CreditCard) }
+    end
   end
 end