feat: deprecate @angular/http in favor of @angular/common/http (#18906)

As of 5.0, @angular/http is deprecated. @angular/common/http will
be the official HTTP API in Angular going forward.

PR Close #18906

packages/common/http/testing/src/module.ts

@@ -18,7 +18,7 @@ import {HttpClientTestingBackend} from './backend';
  *
  * Inject `HttpTestingController` to expect and flush requests in your tests.
  *
- * @experimental
+ * @stable
  */
 @NgModule({
   imports: [