Angular version 6 is not supported (#1062)

Angular version 6 is not supported

Author: ovchinnikov

package.json

@@ -1,6 +1,6 @@
 {
   "name": "devextreme-angular",
-  "version": "19.2.4",
+  "version": "20.1.0",
   "description": "Angular UI and visualization components based on DevExtreme widgets",
   "repository": {
     "type": "git",
@@ -20,9 +20,9 @@
   "license": "MIT",
   "peerDependencies": {
     "devextreme": "20.1-next",
-    "@angular/core": ">6.0.0",
-    "@angular/common": ">6.0.0",
-    "@angular/forms": ">6.0.0"
+    "@angular/core": ">7.0.0",
+    "@angular/common": ">7.0.0",
+    "@angular/forms": ">7.0.0"
   },
   "devDependencies": {
     "@angular/animations": "~7.2.0",

src/core/integration.ts

@@ -1,5 +1,5 @@
 
-import { NgModule, Inject, NgZone, Optional } from '@angular/core';
+import { NgModule, Inject, NgZone, Optional, VERSION } from '@angular/core';
 import { XhrFactory } from '@angular/common/http';
 import * as httpRequest from 'devextreme/core/http_request';
 import { DOCUMENT } from '@angular/common';
@@ -21,6 +21,10 @@ readyCallbacks.inject({
 });
 
 let doInjections = (document: any, ngZone: NgZone, xhrFactory: XhrFactory) => {
+    if (Number(VERSION.major) < 7) {
+        console.warn('Your version of Angular is not supported (https://supportcenter.devexpress.com/ticket/details/t879496). Please update your project to version 7 or later. Please refer to the Angular Update Guide for more information: https://update.angular.io');
+    }
+
     domAdapter.inject({
         _document: document,