Remove mention of deprecated 'jest-native' in docs

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However, jest-native's github repo says that "This package is deprecated and is no longer actively maintained.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."

versioned_docs/version-6.x/testing.md と versioned_docs/version-7.x/testing.md を開き、jest-native に言及している推奨事項を見つけます。非推奨の参照を削除し、React Native Testing Library の現在のガイダンスを反映するようにドキュメントを更新します。両方のページで jest-native が推奨されなくなっていることを確認します。