FIX: Do not duplicate admin sidebar plugin links (#25780)

When hiding/showing the sidebar, as is the case on mobile
and using the toggle in the top left on desktop, we delete
and recreate the ember component on the page. This causes
the `sections` for each sidebar panel to get re-evaluated
every time.

For the admin sidebar, this means that we were constantly
re-adding the plugin links to the sidebar, causing duplication.
This can be fixed by just adding @cached to the getter for
sections.

app/assets/javascripts/discourse/app/lib/sidebar/admin-sidebar.js

@@ -1,3 +1,4 @@
+import { cached } from "@glimmer/tracking";
 import PreloadStore from "discourse/lib/preload-store";
 import { ADMIN_NAV_MAP } from "discourse/lib/sidebar/admin-nav-map";
 import BaseCustomSidebarPanel from "discourse/lib/sidebar/base-custom-sidebar-panel";
@@ -213,6 +214,7 @@ export default class AdminSidebarPanel extends BaseCustomSidebarPanel {
   key = ADMIN_PANEL;
   hidden = true;
 
+  @cached
   get sections() {
     const currentUser = getOwnerWithFallback().lookup("service:current-user");
     const siteSettings = getOwnerWithFallback().lookup("service:site-settings");