FIX: tags need to be an array of strings for SchemaSettingTypeTags (#37643)

Reported here:
https://meta.discourse.org/t/cannot-add-sidebar-via-objects-setting-editor/395567

Objects were getting passed instead of strings, and
`SchemaSettingsObjectValidator` expects an array of strings, not an
array of objects. The fix is similar to what we do for categories here:
https://github.com/discourse/discourse/blob/cb379b8605ad61353482f1d7eb554ed9282ac81c/frontend/discourse/admin/components/schema-setting/types/categories.gjs#L28-L33

frontend/discourse/admin/components/schema-setting/types/tags.gjs

@@ -13,6 +13,10 @@ export default class SchemaSettingTypeTags extends SchemaSettingTypeModels {
     };
   }
 
+  onChange(tags) {
+    return tags.map((tag) => tag.name);
+  }
+
   <template>
     <TagChooser
       @tags={{this.value}}