Add associations to end product establishment for effectuations (#15170)

### Description
PR to take advantage of rails associations to be able to access board grant effectuations easily on a board grant effectuation end product establishment.

app/models/end_product_establishment.rb

@@ -11,7 +11,9 @@
 class EndProductEstablishment < CaseflowRecord
   belongs_to :source, polymorphic: true
   belongs_to :user
+  has_many :request_issues
   has_many :end_product_code_updates
+  has_many :effectuations, class_name: "BoardGrantEffectuation"
 
   # allow @veteran to be assigned to save upstream calls
   attr_writer :veteran
@@ -279,12 +281,6 @@ def generate_tracked_item!
     end
   end
 
-  def request_issues
-    return RequestIssue.none unless source.try(:request_issues)
-
-    source.request_issues.where(end_product_establishment_id: id)
-  end
-
   def associated_rating_cache_key
     "end_product_establishments/#{id}/associated_rating"
   end