Build break fix: set XCode version to 11.2.1

[tom-un]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

The build pipeline was using XCode 11.0. A recent change to the build agents broke the script method we used to select the XCode version.

Our old script method was designed to work in both the public build agents and the custom internal build agents. We no longer use the later, so this change switches to the more conventional method of switching xcode version which is to sudo xcode-select --switch 'path to xcode' and then to use xcodeVersion: default in the XCode@5 tasks.

Microsoft Reviewers: Open in CodeFlow