fix: Change postinstall step to prepare and run prepare on release in package.json

[tboba]

Description

This PR intents to fix error, during the install step on other React Native applications. That's because of the name of the step - it shouldn't be named as postinstall, as method named this way is triggered by yarn v4. This was not an issue before, as we used yarn v1.

Fixes #2108.

Changes

• Changed postinstall name to prepare in package.json
• Added yarn prepare command during the yarn release.

Checklist

• Ensured that CI passes

[kkafar]

That's should be it.