build: run tests against microsoft edge 18

Updates the Microsoft Edge version with the hope of being more stable
than MS Edge 17 which is very flaky on BrowserStack.

Author: devversion

test/karma-browsers.json

@@ -243,7 +243,7 @@
   "BROWSERSTACK_EDGE": {
     "base": "BrowserStack",
     "browser": "Edge",
-    "browser_version": "17.0",
+    "browser_version": "18.0",
     "os": "Windows",
     "os_version": "10"
   },