Fix react-native-oss-ios by bumping Xcode to 12.4.0 (#31554)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/31554

## Changes
This diff bumps Xcode to 12.4.0 both internally, and in Circle CI.

- According to [Circle CI docs](https://circleci.com/docs/2.0/testing-ios/#supported-xcode-versions), their environments support Xcode 12.4.0.
- According to the [Apple Docs](https://developer.apple.com/support/xcode/), Xcode 12.4.0 matches iOS 14.4.

Changelog: [Internal]

Reviewed By: PeteTheHeat

Differential Revision: D28547839

fbshipit-source-id: c9edde6a5825ae597bf56225d1b017c3af0f52ae