DEV: Only show plugin tab for admin (#20841)

As part of another regression, we realized that the plugins tab is visible to moderators, but they cannot interact with anything inside without triggering authorization errors.

This change hides the plugin tab for non-admin users.

app/assets/javascripts/admin/addon/templates/admin.hbs

@@ -27,8 +27,8 @@
             {{#if this.siteSettings.enable_backups}}
               <NavItem @route="admin.backups" @label="admin.backups.title" />
             {{/if}}
+            <NavItem @route="adminPlugins" @label="admin.plugins.title" />
           {{/if}}
-          <NavItem @route="adminPlugins" @label="admin.plugins.title" />
           <PluginOutlet @name="admin-menu" />
         </ul>
       </div>