Remove iOS 11 availability check (#32488)

Summary:
This pull request aims to remove iOS 11 availability check which is no longer needed.

The minimum iOS deployment target for React Native is iOS 11 but we still have iOS 11 version check like below.

```
if (available(iOS 11.0, *)) {
```

This is a continuation pull request of https://github.com/facebook/react-native/pull/32151

## Changelog

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[iOS] [Changed] - Remove iOS 11 availability check

Pull Request resolved: https://github.com/facebook/react-native/pull/32488

Reviewed By: yungsters

Differential Revision: D32006312

Pulled By: ryancat

fbshipit-source-id: 0ee6579e433a15d3d220a52d2ccd6931b0513971