Remove mention of deprecated 'jest-native' in docs

[sebastiancarlos]

The following pages contain a reference to jest-native which is deprecated:

• versioned_docs/version-6.x/testing.md
• versioned_docs/version-7.x/testing.md

The text says: "We recommend using React Native Testing Library along with jest-native to write your tests."

However, jest-native's github repo says that "This package is deprecated and is no longer actively maintained. We encourage you to migrate to React Native Testing Library, v12.4 or later, which includes modern built-in Jest matchers based on the matchers for this repository."