FEATURE: Support designating multiple groups as mods on category (#28655)

Currently, categories support designating only 1 group as a moderation group on the category. This commit removes the one group limitation and makes it possible to designate multiple groups as mods on a category.

Internal topic: t/124648.

spec/models/post_action_spec.rb

@@ -99,7 +99,7 @@ RSpec.describe PostAction do
       before do
         SiteSetting.enable_category_group_moderation = true
         group.update!(messageable_level: Group::ALIAS_LEVELS[:nobody])
-        post.topic.category.update!(reviewable_by_group_id: group.id)
+        Fabricate(:category_moderation_group, category: post.topic.category, group:)
       end
 
       it "notifies via pm" do