Update signalR to core version

src/Umbraco.Web.UI.Client/package.json

@@ -10,6 +10,7 @@
     "watch": "gulp watch"
   },
   "dependencies": {
+    "@microsoft/signalr": "^3.1.8",
     "ace-builds": "1.4.2",
     "angular": "1.8.0",
     "angular-animate": "1.7.5",
@@ -40,7 +41,6 @@
     "ng-file-upload": "12.2.13",
     "nouislider": "14.6.1",
     "npm": "^6.14.7",
-    "signalr": "2.4.0",
     "spectrum-colorpicker2": "2.0.3",
     "tinymce": "4.9.11",
     "typeahead.js": "0.11.1",