Upgrade tests to Xcode 11.3.1 (#28498)

Summary:
Upgrade Sandcastle and Circle CI tests to use Xcode 11.3.1 across the board.

Pull Request resolved: https://github.com/facebook/react-native/pull/28498
Pull Request resolved: https://github.com/facebook/react-native/pull/28501

Changelog:
[Internal] - Use Xcode 11.3.1 in iOS tests

Reviewed By: fkgozali

Differential Revision: D20821844

fbshipit-source-id: b250ca82bdf2c9fb7faa765d3e2433eb46efd692

.circleci/config.yml

@@ -50,7 +50,7 @@ executors:
   reactnativeios:
     <<: *defaults
     macos:
-      xcode: "11.3.0"
+      xcode: &_XCODE_VERSION "11.3.1"
 
 # -------------------------
 #        COMMANDS

scripts/.tests.env

@@ -1,5 +1,5 @@
 # These should match the recommended versions listed in
-# https://reactnative.dev/docs/getting-started.html
+# https://reactnative.dev/docs/environment-setup
 
 
 ## ANDROID ##