DEV: Add problem check informing about admin layout deprecation (#30685)

We want to avoid surprises when we make the new admin sidebar baseline, so in addition to announcements, we're also adding a problem check that informs you if you don't have it enabled for any group yet.

app/services/problem_check/admin_sidebar_deprecation.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ProblemCheck::AdminSidebarDeprecation < ProblemCheck::ProblemCheck
+  self.priority = "low"
+
+  def call
+    return no_problem if SiteSetting.admin_sidebar_enabled_groups.present?
+
+    problem
+  end
+end