docs(angular): downgrade v9 stackblitz playground to angular 21 for node compatibility

Author: ShaneK

static/code/stackblitz/v9/angular/main.ts

@@ -8,8 +8,7 @@ import { AppComponent } from './app/app.component';
 
 bootstrapApplication(AppComponent, {
   providers: [
-    // Angular 21+ defaults to zoneless change detection. The playground opts into
-    // Zone.js so the embedded examples render without per-example signal wiring.
+    // Angular 21 defaults to zoneless change detection; Ionic requires zone-based.
     provideZoneChangeDetection(),
     { provide: RouteReuseStrategy, useClass: IonicRouteStrategy },
     provideIonicAngular(),