Update LFK script to generate constant for migration

scripts/decomposition/generate-loose-foreign-key

@@ -164,18 +164,20 @@ def generate_migration(definition)
     class Remove#{definition.to_table.camelcase}#{definition.from_table.camelcase}#{definition.column.camelcase}Fk < Gitlab::Database::Migration[2.1]
       disable_ddl_transaction!
 
+      FOREIGN_KEY_NAME = "#{definition.name}"
+
       def up
-        return unless foreign_key_exists?(:#{definition.from_table}, :#{definition.to_table}, name: "#{definition.name}")
+        return unless foreign_key_exists?(:#{definition.from_table}, :#{definition.to_table}, name: FOREIGN_KEY_NAME)
 
         with_lock_retries do
           remove_foreign_key_if_exists(:#{definition.from_table}, :#{definition.to_table},
-            name: "#{definition.name}", reverse_lock_order: true)
+            name: FOREIGN_KEY_NAME, reverse_lock_order: true)
         end
       end
 
       def down
         add_concurrent_foreign_key(:#{definition.from_table}, :#{definition.to_table},
-          name: "#{definition.name}", column: :#{definition.column},
+          name: FOREIGN_KEY_NAME, column: :#{definition.column},
           target_column: :#{definition.primary_key}, on_delete: :#{definition.on_delete})
       end
     end