Bump Xcode to 11.2.1

[hramos]

Summary:
Use Xcode 11 when testing iOS changes.

testNotUTF8Convertible disabled as NSString's -initWithBytes:length:encoding: segfaults on iOS 13.

Differential Revision: D18849584

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D18849584

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D18849584

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D18849584

[pull-bot]

	Warnings
⚠️	🔒 package.json - Changes were made to package.json. This will require a manual import by a Facebook employee.

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Generated by 🚫 dangerJS against be54e7f

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D18849584

[react-native-bot]

This pull request was successfully merged by @hramos in 9fb2ca0.

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