Deprecate Spree::TaxRate#adjust method

We are not using this method in the codebase and now we have
Spree::Tax::OrderAdjuster class which takes care of applying
taxes to an order. That's the new default way and we should
start printing a notice into apps/extensions that are still
using the old way.

core/app/models/spree/tax_rate.rb

@@ -88,7 +88,11 @@ class TaxRate < Spree::Base
     scope :included_in_price, -> { where(included_in_price: true) }
 
     # Creates necessary tax adjustments for the order.
+    #
+    # @deprecated Please use `Spree::Tax::OrderAdjuster#adjust!` instead
     def adjust(_order_tax_zone, item)
+      Spree::Deprecation.warn("`Spree::TaxRate#adjust` is deprecated. Please use `Spree::Tax::OrderAdjuster#adjust!` instead.", caller)
+
       amount = compute_amount(item)
 
       item.adjustments.create!(

core/spec/models/spree/tax_rate_spec.rb

@@ -153,6 +153,7 @@
 
     describe 'adjustments' do
       before do
+        expect(Spree::Deprecation).to receive(:warn)
         tax_rate.adjust(nil, item)
       end