FIX: Use plugin category name for plugin list (#24477)

Followup to e37fb3042d

Some plugins like discourse-ai and discourse-saml do not
nicely change from kebab-case to Title Case (e.g. Ai, Saml),
and anyway this method of getting the plugin name is not
translated either.

Better to use the plugin setting category if it exists,
since that is written by a human and is translated.

app/assets/javascripts/admin/addon/templates/site-settings.hbs

@@ -29,10 +29,15 @@
   </div>
 </div>
 
-<div class="admin-nav pull-left">
+<div class="admin-nav admin-site-settings-category-nav pull-left">
   <ul class="nav nav-stacked">
     {{#each this.visibleSiteSettings as |category|}}
-      <li class={{category.nameKey}}>
+      <li
+        class={{concat-class
+          "admin-site-settings-category-nav__item"
+          category.nameKey
+        }}
+      >
         <LinkTo
           @route="adminSiteSettingsCategory"
           @model={{category.nameKey}}