Remove Spree::Adjustment#promotion_code_id from core

This moves the column into solidus_legacy_promotions, which is the only
place it's used at.

core/db/migrate/20160101010000_solidus_one_four.rb

@@ -90,14 +90,12 @@ def up
       t.datetime "updated_at", precision: 6
       t.integer "order_id", null: false
       t.boolean "included", default: false
-      t.integer "promotion_code_id"
       t.integer "adjustment_reason_id"
       t.boolean "finalized", default: false, null: false
       t.index ["adjustable_id", "adjustable_type"], name: "index_spree_adjustments_on_adjustable_id_and_adjustable_type"
       t.index ["adjustable_id"], name: "index_adjustments_on_order_id"
       t.index ["eligible"], name: "index_spree_adjustments_on_eligible"
       t.index ["order_id"], name: "index_spree_adjustments_on_order_id"
-      t.index ["promotion_code_id"], name: "index_spree_adjustments_on_promotion_code_id"
       t.index ["source_id", "source_type"], name: "index_spree_adjustments_on_source_id_and_source_type"
     end
 

legacy_promotions/db/migrate/20240621100123_add_promotion_code_id_to_spree_adjustments.rb

@@ -0,0 +1,10 @@
+class AddPromotionCodeIdToSpreeAdjustments < ActiveRecord::Migration[5.0]
+  def up
+    unless column_exists?(:spree_adjustments, :promotion_code_id)
+      add_column :spree_adjustments, :promotion_code_id, :integer
+    end
+    unless index_exists?(:spree_adjustments, :promotion_code_id)
+      add_index :spree_adjustments, :promotion_code_id, name: "index_spree_adjustments_on_promotion_code_id"
+    end
+  end
+end

legacy_promotions/spec/models/spree/adjustment_spec.rb

@@ -9,6 +9,7 @@
 
   let(:adjustment) { Spree::Adjustment.create!(label: 'Adjustment', adjustable: order, order: order, amount: 5) }
 
+  it { is_expected.to respond_to(:promotion_code) }
   context '#recalculate' do
     subject { adjustment.recalculate }
     let(:adjustment) do