Simplify CurrentAttribute.instance delegation

Follow-up to [#50676][]

Instead of relying on code generation, call a corresponding [delegate][]
method on the `.singleton_class`.

[#50676]: #50676
[delegate]: https://edgeapi.rubyonrails.org/classes/Module.html#method-i-delegate

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

activesupport/lib/active_support/current_attributes.rb

@@ -125,22 +125,7 @@ def attribute(*names)
           end
         end
 
-        ActiveSupport::CodeGenerator.batch(singleton_class, __FILE__, __LINE__) do |owner|
-          names.each do |name|
-            owner.define_cached_method(name, namespace: :current_attributes_delegation) do |batch|
-              batch <<
-                "def #{name}" <<
-                "instance.#{name}" <<
-                "end"
-            end
-            owner.define_cached_method("#{name}=", namespace: :current_attributes_delegation) do |batch|
-              batch <<
-                "def #{name}=(value)" <<
-                "instance.#{name} = value" <<
-                "end"
-            end
-          end
-        end
+        singleton_class.delegate(*names.flat_map { |name| [name, "#{name}="] }, to: :instance, as: self)
       end
 
       # Calls this callback before #reset is called on the instance. Used for resetting external collaborators that depend on current values.