build: update Stackblitz template to v21

Updates the Stackblitz template to v21 and fixes an issue with the dev server.

Author: crisbeto

docs/src/assets/stackblitz/angular.json

@@ -21,7 +21,7 @@
       "prefix": "app",
       "architect": {
         "build": {
-          "builder": "@angular-devkit/build-angular:application",
+          "builder": "@angular/build:application",
           "options": {
             "outputPath": "dist/example-app",
             "index": "src/index.html",
@@ -65,7 +65,7 @@
           "defaultConfiguration": "production"
         },
         "serve": {
-          "builder": "@angular-devkit/build-angular:dev-server",
+          "builder": "@angular/build:dev-server",
           "configurations": {
             "production": {
               "buildTarget": "example-app:build:production"
@@ -76,14 +76,8 @@
           },
           "defaultConfiguration": "development"
         },
-        "extract-i18n": {
-          "builder": "@angular-devkit/build-angular:extract-i18n",
-          "options": {
-            "buildTarget": "example-app:build"
-          }
-        },
         "test": {
-          "builder": "@angular-devkit/build-angular:karma",
+          "builder": "@angular/build:karma",
           "options": {
             "main": "src/test.ts",
             "polyfills": ["zone.js", "zone.js/testing"],