DEV: Convert min_trust_to_flag_posts setting to groups (#24864)

We're changing the implementation of trust levels to use groups. Part of this is to have site settings that reference trust levels use groups instead. It converts the min_trust_to_flag_posts site setting to flag_post_allowed_groups.

Note: In the original setting, "posts" is plural. I have changed this to "post" singular in the new setting to match others.

plugins/discourse-local-dates/spec/system/local_dates_spec.rb

@@ -2,7 +2,7 @@
 
 describe "Local dates", type: :system do
   fab!(:topic)
-  fab!(:current_user) { Fabricate(:user) }
+  fab!(:current_user) { Fabricate(:user, refresh_auto_groups: true) }
   let(:year) { Time.zone.now.year + 1 }
   let(:month) { Time.zone.now.month }
   let(:bookmark_modal) { PageObjects::Modals::Bookmark.new }