FIX: public sidebar sections belong to system user (#20972)

Before, public sidebar sections were belonging to admin. However, a better choice is system user.

spec/models/sidebar_section_spec.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+RSpec.describe SidebarSection do
+  fab!(:user) { Fabricate(:user) }
+  fab!(:sidebar_section) { Fabricate(:sidebar_section, user: user) }
+
+  it "uses system user for public sections" do
+    expect(sidebar_section.user_id).to eq(user.id)
+    sidebar_section.update!(public: true)
+    expect(sidebar_section.user_id).to eq(Discourse.system_user.id)
+  end
+end