fix: 🩹 Updated README.md and example app

[Sahil-Simform]

Description

🩹Fix: Updated README.md. and example app.

• Removed new preview for now
• Updated example app as per the requirements
• Added (unreleased) in migration guild for 4.0.0 version

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.