Update README.md - iOS support is 13.4 (#38108)

Summary:
According to the podfiles, the minimum version is 13.4

## Changelog:

[General] [Fixed] - Updated docs to match iOS 13.4 minimum requirement.

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Pull Request resolved: https://github.com/facebook/react-native/pull/38108

Reviewed By: jacdebug

Differential Revision: D47093313

Pulled By: blakef

fbshipit-source-id: f59b1be3a6608082fd680e42396aff9a269c77a0