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/client.ts

@@ -46,7 +46,7 @@ function addBody<T>(
 }
 
 /**
- * @experimental
+ * @stable
  */
 export type HttpObserve = 'body' | 'events' | 'response';
 
@@ -57,7 +57,7 @@ export type HttpObserve = 'body' | 'events' | 'response';
  * Each request method has multiple signatures, and the return type varies according to which
  * signature is called (mainly the values of `observe` and `responseType`).
  *
- * @experimental
+ * @stable
  */
 @Injectable()
 export class HttpClient {