Merge forwardport of #11134 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11134.patch (created by @Zifius) based on commit(s):
  1. 70379e3

Fixed GitHub Issues in 2.3-develop branch:
  - #10775: 404 Forbidden sounds not right (reported by @irinikp)

app/code/Magento/Backend/Controller/Adminhtml/Noroute/Index.php

@@ -34,7 +34,7 @@ public function execute()
     {
         /** @var \Magento\Backend\Model\View\Result\Page $resultPage */
         $resultPage = $this->resultPageFactory->create();
-        $resultPage->setStatusHeader(404, '1.1', 'Forbidden');
+        $resultPage->setStatusHeader(404, '1.1', 'Not Found');
         $resultPage->setHeader('Status', '404 File not found');
         $resultPage->addHandle('adminhtml_noroute');
         return $resultPage;