chore: update reference in cross-platform test workflow from v8 to main

[itaybre]

📜 Description

Updates RN branch to main

#skip-changelog

💡 Motivation and Context

React native just released v8, so we don't need to use a special branch anymore.

💚 How did you test it?

📝 Checklist

You have to check all boxes before merging:

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

Closes #7450

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

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