Move Taxon -> Promotion Rule association to legacy promotions

This was forgotten when extracting the legacy_promotions gem. Also adds
a dependent: :destroy.

(cherry picked from commit 8f5f4ff)

Author: mamhoff

core/app/models/spree/taxon.rb

@@ -13,9 +13,6 @@ class Taxon < Spree::Base
     has_many :classifications, -> { order(:position) }, dependent: :delete_all, inverse_of: :taxon
     has_many :products, through: :classifications
 
-    has_many :promotion_rule_taxons
-    has_many :promotion_rules, through: :promotion_rule_taxons
-
     before_save :set_permalink
     after_update :update_child_permalinks, if: :saved_change_to_permalink?
 

legacy_promotions/app/patches/models/solidus_legacy_promotions/spree_taxon_patch.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module SolidusLegacyPromotions
+  module SpreeTaxonPatch
+    def self.prepended(base)
+      base.has_many :promotion_rule_taxons, dependent: :destroy
+      base.has_many :promotion_rules, through: :promotion_rule_taxons
+    end
+
+    ::Spree::Taxon.prepend self
+  end
+end