FIX: stop memoize PostActionTypes (#28005)

Memoizing all_flags on PostActionType was a mistake. This commit brings back the cache on the serialize level.

spec/models/post_action_type_spec.rb

@@ -1,6 +1,24 @@
 # frozen_string_literal: true
 
 RSpec.describe PostActionType do
+  describe "Callbacks" do
+    describe "#expiry_cache" do
+      it "should clear the cache on save" do
+        cache = ApplicationSerializer.fragment_cache
+
+        cache["post_action_types_#{I18n.locale}"] = "test"
+        cache["post_action_flag_types_#{I18n.locale}"] = "test2"
+
+        PostActionType.new(name_key: "some_key").save!
+
+        expect(cache["post_action_types_#{I18n.locale}"]).to eq(nil)
+        expect(cache["post_action_flag_types_#{I18n.locale}"]).to eq(nil)
+      ensure
+        ApplicationSerializer.fragment_cache.clear
+      end
+    end
+  end
+
   describe "#types" do
     context "when verifying enum sequence" do
       before { @types = PostActionType.types }