FIX: Do not double-highlight admin plugin links in sidebar (#25808)

I was using adminPlugins instead of adminPlugins.index for the Installed Plugins
link in admin sidebar, which was causing Ember to highlight the main link and a plugin
link at the same time.

We only want to highlight the top level Installed Plugins link if we are on that page,
not if we are on e.g. the chat plugin admin route.

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

@@ -3,12 +3,11 @@ import getURL from "discourse-common/lib/get-url";
 export const ADMIN_NAV_MAP = [
   {
     name: "plugins",
-    route: "adminPlugins.index",
     label: "admin.plugins.title",
     links: [
       {
         name: "admin_installed_plugins",
-        route: "adminPlugins",
+        route: "adminPlugins.index",
         label: "admin.plugins.installed",
         icon: "puzzle-piece",
       },

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

@@ -90,7 +90,9 @@ function defineAdminSection(adminNavSectionData) {
     get links() {
       return this.sectionLinks.map(
         (sectionLinkData) =>
-          new SidebarAdminSectionLink({ adminSidebarNavLink: sectionLinkData })
+          new SidebarAdminSectionLink({
+            adminSidebarNavLink: sectionLinkData,
+          })
       );
     }
 
@@ -216,8 +218,12 @@ export default class AdminSidebarPanel extends BaseCustomSidebarPanel {
 
   @cached
   get sections() {
-    const currentUser = getOwnerWithFallback().lookup("service:current-user");
-    const siteSettings = getOwnerWithFallback().lookup("service:site-settings");
+    const currentUser = getOwnerWithFallback(this).lookup(
+      "service:current-user"
+    );
+    const siteSettings = getOwnerWithFallback(this).lookup(
+      "service:site-settings"
+    );
     if (!currentUser.use_admin_sidebar) {
       return [];
     }