Update simulator matrix

Pick a couple iPad and iPhone values default-enabled in Xcode 11 that also exist in Xcode 10

.github/workflows/build-ios-latest.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
         matrix:
             config: [release, debug]
-            simulator: ["'iPhone 8'", "'iPhone X'"]
+            simulator: ["'iPhone 8'", "'iPad Air (3rd generation)'"]
     steps:
     - uses: actions/checkout@v1
     - name: build