Ignore Arcade and Helix

Also ignore Arcade and Helix as dependabot corrupts the configuration file.

Author: martincostello

.github/dependabot.yml

@@ -20,3 +20,5 @@ updates:
     - dependency-name: "Microsoft.AspNetCore.TestHost"
       update-types: ["version-update:semver-major"]
     - dependency-name: "Microsoft.IdentityModel.Protocols"
+    - dependency-name: "Microsoft.DotNet.Arcade.Sdk"
+    - dependency-name: "Microsoft.DotNet.Helix.Sdk"