Bump Xcode to 11.2.1 (#27434)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/27434
Pull Request resolved: https://github.com/facebook/react-native/pull/27295

- Use Xcode version 11.2.1 in both internal and external tests.
- Update snapshot reference images to use iPhone 8 test device.

Xcode 11.3.0 will be available in Sandcastle in the next week or so.

Changelog:

[Internal] - Bump Xcode version used in tests to 11.2.1.
[Internal] - Update iOS Snapshots.

Reviewed By: cs01

Differential Revision: D18849584

fbshipit-source-id: 9e3b92f080a4dc6a15dac0535b7562ff6a5c08a9