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/src/request.ts

@@ -73,7 +73,7 @@ function isFormData(value: any): value is FormData {
  * assumed to be immutable. To modify a `HttpRequest`, the `clone`
  * method should be used.
  *
- * @experimental
+ * @stable
  */
 export class HttpRequest<T> {
   /**