fix build

Author: morganchen12

.github/workflows/firestore-ios.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: macOS-latest
     strategy:
         matrix:
-          destination: ['platform=iOS Simulator,OS=13.5,name=iPhone 11']
+          destination: ['platform=iOS Simulator,OS=latest,name=iPhone 11']
     steps:
       - name: Checkout
         uses: actions/checkout@master