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.

config/locales/server.en.yml

@@ -1722,6 +1722,7 @@ en:
       google_analytics_version: "Your Discourse is currently using Google Analytics 3, which will no longer be supported after July 2023. <a href='https://meta.discourse.org/t/260498'>Upgrade to Google Analytics 4</a> now to continue receiving valuable insights and analytics for your website's performance."
       category_style_deprecated: "Your Discourse is currently using a deprecated category style which will be removed before the final beta release of Discourse 3.2. Please refer to <a href='https://meta.discourse.org/t/282441'>Moving to a Single Category Style Site Setting</a> for instructions on how to keep your selected category style."
       maxmind_db_configuration: 'The server has been configured to use MaxMind databases for reverse IP lookups but a valid MaxMind account ID has not been configured which may result in MaxMind databases failing to download in the future. <a href="https://meta.discourse.org/t/configure-maxmind-for-reverse-ip-lookups/173941" target="_blank">See this guide to learn more</a>.'
+      admin_sidebar_deprecation: "The old admin layout is deprecated in favour of the new <a href='https://meta.discourse.org/t/introducing-experimental-admin-sidebar-navigation/289281'>sidebar layout</a> and will be removed in the next release. You can <a href='%{base_path}/admin/config/navigation?filter=admin%20sidebar'>configure</a> the new sidebar layout now to opt in before that."
     back_from_logster_text: "Back to site"
 
   site_settings: