Switch off autocomplete

see #17783 (comment)

Author: flancer64

app/code/Magento/Backend/view/adminhtml/templates/admin/login.phtml

@@ -27,7 +27,7 @@
                        value=""
                        data-validate="{required:true}"
                        placeholder="<?= /* @escapeNotVerified */ __('user name') ?>"
-                       autocomplete="on"
+                       autocomplete="off"
                     />
             </div>
         </div>
@@ -43,7 +43,7 @@
                        data-validate="{required:true}"
                        value=""
                        placeholder="<?= /* @escapeNotVerified */ __('password') ?>"
-                       autocomplete="current-password"
+                       autocomplete="off"
                     />
             </div>
         </div>