docs: modernize Mongoose examples using async/await

[Vikash9546]

This PR updates the Mongoose examples in the README.md. As of Mongoose 7.x+, Model.findOne() and Model.findById() no longer accept callbacks. The documentation examples have been updated to use modern async/await syntax to prevent confusion for new users setting up Passport alongside modern Mongoose.

Changes Made

• Updated the LocalStrategy example in the Strategies section to use a try...catch block with await User.findOne().
• Updated the passport.deserializeUser example in the Sessions section to use a try...catch block with await User.findById().