Revert "Update dependency @vaadin/vaadin-grid to v6 (#2380)" (#2415)

This reverts commit 26fae7f.

Author: KyleJu

webapp/package-lock.json

@@ -71,7 +71,7 @@
     "@polymer/polymer": "3.4.1",
     "@vaadin/vaadin-context-menu": "5.0.0",
     "@vaadin/vaadin-date-picker": "4.4.1",
-    "@vaadin/vaadin-grid": "6.0.0",
+    "@vaadin/vaadin-grid": "5.7.7",
     "@webcomponents/webcomponentsjs": "2.5.0",
     "pluralize": "8.0.0"
   }