CI - Update Xcode to 12+ for E2E tests

[mikehardy]

The GitHub Actions lag a little bit, they are still on 11.7: https://github.com/actions/virtual-environments/blob/main/images/macos/macos-10.15-Readme.md#xcode

It may be that Xcode 12 will surface a problem: invertase/firestore-ios-sdk-frameworks#15

https://github.com/maxim-lobanov/setup-xcode can be used to switch