Update some auth tests to Xcode 16 (#13745)

Author: paulb777

.github/workflows/auth.yml

@@ -163,7 +163,7 @@ jobs:
         scripts/decrypt_gha_secret.sh scripts/gha-encrypted/AuthSample/Credentials.swift.gpg \
           FirebaseAuth/Tests/SampleSwift/SwiftApiTests/Credentials.swift "$plist_secret"
     - name: Xcode
-      run: sudo xcode-select -s /Applications/Xcode_15.4.app/Contents/Developer
+      run: sudo xcode-select -s /Applications/Xcode_16.app/Contents/Developer
     - uses: nick-fields/retry@v3
       with:
         timeout_minutes: 120