FIX: Updating presence status in readonly mode should fail gracefully (#24333)

app/controllers/presence_controller.rb

@@ -42,6 +42,8 @@ class PresenceController < ApplicationController
   end
 
   def update
+    raise Discourse::ReadOnly if @readonly_mode
+
     client_id = params[:client_id]
     if !client_id.is_a?(String) || client_id.blank?
       raise Discourse::InvalidParameters.new(:client_id)