switch Readme version badge source to shields.io (#30411)

Summary:
Lately there are some loading issues for the `fury.io` badges service which results in missing image and alt text visible in the Readme.

This PR fixes that issue by switching to `shields.io` version badge - badge service that has been used for few other badges already.

## Changelog

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https://github.com/facebook/react-native/wiki/Changelog
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[Internal] [Changed] - change Readme npm version badge to `shields.io`

Pull Request resolved: https://github.com/facebook/react-native/pull/30411

Test Plan: All badges will appear in the Readme file. {emoji:1f642}

Reviewed By: cpojer

Differential Revision: D25040061

Pulled By: hramos

fbshipit-source-id: 2ef6b94ff39ec21573b7e58318aba653fd716037