build: promote @angular-devkit/build-angular to stable

Author: alan-agius4

packages/angular_devkit/build_angular/package.json

@@ -2,7 +2,6 @@
   "name": "@angular-devkit/build-angular",
   "version": "0.0.0",
   "description": "Angular Webpack Build Facade",
-  "experimental": true,
   "main": "src/index.js",
   "typings": "src/index.d.ts",
   "builders": "builders.json",

packages/schematics/angular/utility/latest-versions.ts

@@ -18,7 +18,7 @@ export const latestVersions = {
   // For our e2e tests, these versions must match the latest tag present on the branch.
   // During RC periods they will not match the latest RC until there's a new git tag, and
   // should not be updated.
-  DevkitBuildAngular: '~0.1200.0-next.9',
+  DevkitBuildAngular: '~12.0.0-next.9',
 
   ngPackagr: '^12.0.0-next.8',
 };