Add admin notifications for new Mastodon versions (#26582)

app/controllers/admin/software_updates_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Admin
+  class SoftwareUpdatesController < BaseController
+    before_action :check_enabled!
+
+    def index
+      authorize :software_update, :index?
+      @software_updates = SoftwareUpdate.all.sort_by(&:gem_version)
+    end
+
+    private
+
+    def check_enabled!
+      not_found unless SoftwareUpdate.check_enabled?
+    end
+  end
+end